Question

For what values of x is the following equation satisfied : 3x+9 = 21+7x ?

Answer 1

$3x+9 = 21+7x\\ 4x=-12\\ x=-3$

Answer 2

First of all, the first power of 'x' is the highest power in the whole equation,
so we only expect one solution.

You said that 3x+9 = 21+7x

Subtract 3x from each side:

9 = 21 + 4x

Subtract 21 from each side:

-12 = 4x

Divide each side by 4 :

-3 = x