How do I solve the following Pre-Algebra equation: x = -2x - 21 I am a parent of a 7th grader and its been a LONG time since I did this stuff, looking for more than the answer so I can understand...

How do I solve the following Pre-Algebra equation:

x = -2x - 21

I am a parent of a 7th grader and its been a LONG time since I did this stuff, looking for more than the answer so I can understand the theory / logic behind how the answer is found.

Thank You!

Asked on by abartron

8 Answers | Add Yours

baxthum8's profile pic

baxthum8 | High School Teacher | (Level 3) Associate Educator

Posted on

Solve `x = -2x - 21.`

In order to solve a 1-variable equation we must get all the variables on 1 side and constants on the other.

In this case I need to move the (-2x) to the left side of the equal sign to get the "x's" together.  In order to do this I must add 2x.

`x + (2x) = -2x - 21 + (2x)`

`3x = -21` Now I need to isolate the variable by getting rid of the 3 or making 1 "x".  If I divide both sides by 3, this will make 1 "x".

`(3x)/3 = (-21)/3`

`x = -7`

The solution is x = -7.

CHECK----------------------------------------------------------
I can check my solution by substituting -7 back in for x and make sure both sides of equality come out equal to each other.

`-7 = -2(-7) - 21`

`-7 = 14 - 21`

`-7 = -7`

Since I got the same thing on each side, I'm sure my solution of -7 is true.

malkaam's profile pic

malkaam | Student, Undergraduate | (Level 1) Valedictorian

Posted on

x = -2x - 21

The main purpose of this is to find the value of the variable x, to do that all the similar values are to brought to one side. Also remember this rule of algebra that what happens to one side has to happen to the other side as well, therefore

x = -2x - 21

x + 2x = -2x - 21 + 2x           Add 2x to both sides

3x = -21                                

3x/3 = -21/3                          Both sides divided by 3

x = -7 Answer.

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tonys538 | Student, Undergraduate | (Level 1) Valedictorian

Posted on

The equation x = -2x - 21 has to be solved.

This is done by isolating x to one side of the equation and the numeric terms to the other side. Adding or subtracting the same number to both sides of the equation does not alter the equation. The same is true for multiplying both sides or dividing both sides by the same number.

x = -2x - 21

Add 2x to both the sides

x + 2x = -2x - 21 + 2x

3x = -21

Divide both the sides by 3

`(3x)/3 = (-21)/3`

x = -7

The solution of the equation is x = -7

givingiswinning's profile pic

givingiswinning | Student, Grade 10 | (Level 1) Valedictorian

Posted on

x = -2x - 21 

add 2x on both sides 

you should get

3x = -21

divide by 3 to get " x " alone

x = -7 which is your answer

jess1999's profile pic

jess1999 | Student, Grade 9 | (Level 1) Valedictorian

Posted on

x = -2x - 21 

First we have to add 2x on both sides since we have to try to get " x " on the same side

By adding 2x on both sides, you should get

3x = -21 now divide both sides by 3 because we have to get " x " alone

By dividing by 3 on both sides, you should get

x = -7 which is your answer

atyourservice's profile pic

atyourservice | Student, Grade 11 | (Level 3) Valedictorian

Posted on

`x = -2x - 21`

add 2x to both sides as we are looking for the value of x

`x+2x = -2x+2x-21`

as you added 2x it cancels out the 2x on the right so you end up with

`3x = -21`

now divide by 3 as again we are trying to get x by itself

`(3x) / 3 = (-21) /3`

x = -7 the answer is -7

aruv's profile pic

aruv | High School Teacher | (Level 2) Valedictorian

Posted on

`x=-2x-21`

In this equation x is variable and 21 is constant. We wish to find the value of x. In equation there are two sides namely left hand side and right hand side. Keep variable one side and constant another side. When you are changing sides, negative changes to positive and division changes to multiplication and vice versa.

`x=-2x-21`

`x+2x=-2x-21+2x`

`3x=-2x+2x-21`

`3x=-21`

`(3x)/3=(-21)/3`

`x=-7`

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