2x+8=x-9

$$\begin{array}{rrlll}\\\\
2x+8&=&x-9\qquad&(-x\;on\;each\;side)\\\\
2x+8(-x)&=&x-9(-x)\\\\
x+8&=&-9\qquad&(-8\;on\;each\;side)\\\\
x+8(-8)&=&-9(-8)\\\\
x&=&-17
\end{array}$$

Sorry! 

Subtract x from both sides: x+8=-9.

Subtract 8 from both sides: x=-17.

You got into a mess MG,    

x-x=0   !!

and

2x-x+17=x+17