First, subtract 12 and get it on the other side.
So you will get this:
-4/5(x+15)=-8
Next, you'll want to distribute the -4/5:
-4/5x-12=-8
Add 12
-4/5x=4
Multiply -4/5 (Usually you would divide here, but since it's a fraction, multiplying is simpler)
x=-3.2 (or -16/5)