The same variable can not be on both sides of the equation.
Add 7s to both sides so that the variable is only on he right side.
12=-2s
Divide each side by -2
s=-6
To check your answer, substitute -6 in for s
-7(-6)+12=-9(-6)
42+12=54
54=54
s=-6
Hope this helped! Tell me if you want me to help with anything else or explain this in a different way :)