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