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!

Expert Answers

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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.

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.

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