The given equation is -3x=21.

To check if x=7 is a solution, we substitute 7 for x; if the resulting statement is true, 7 is a solution, if not 7 is not a solution.

**-3(7)=-21 which is not equal to 21, so 7 is not a solution.**

To solve, we multiply both sides of the equation by the multiplicative inverse of -3, `-1/3` . (You can also think of this as an "undo" operation of multiplication -- divide both sides by -3.) So

-3x=21

`(-1/3)-3x=(-1/3)21` Using the multiplication property of equality

`1x=-7` Using teh multiplicative inverse property

x=-7 Using the multiplicative identity property.

**Thus the correct solution is -7.**