Answer :

konrad509
[tex] 3x+9 = 21+7x\\ 4x=-12\\ x=-3[/tex]
AL2006
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