Question 868358
Add 7s to both sides to get



-7s + 12 = -9s



-7s + 12 <font color="red">+7s</font> = -9s <font color="red">+7s</font>



12 = -2s 



Notice how the -7s and the +7s on the left side add to 0s or just 0 (and it goes away).