You need to get "a" by itself:
[tex]\frac{a}{2}-4=-2[/tex] Add 4 on both sides
[tex]\frac{a}{2}-4+4=-2+4[/tex]
[tex]\frac{a}{2}=2[/tex] Multiply 2 on both sides to get "a" by itself
[tex](2)\frac{a}{2}=2(2)[/tex]
a = 4
