Answer :
I'm not sure if the s was supposed to be an f, but if it was an f, then:
f=3f-24
-3f -3f (what you do to one side you do to the other)
-----------
-2f=-24
Divide both sides by -2
f=12
f=3f-24
-3f -3f (what you do to one side you do to the other)
-----------
-2f=-24
Divide both sides by -2
f=12