Not too sure if i can tell u much but the answer on my test is B
I will draw a place value chart on paper than write the numbers
Distribute
-12 -6a = -48
Add 12 to isolate -6a
-6a = -36
Divide by -6 on both sides to isolate a
a = 6
So firstly, subtract x on both sides: 
Next, divide -2 on both sides, and your answer will be y = 1/2x - 3/2 , or the last option.