a = 9
Explanation:
We are given -12 = -2a +6
so to find the value of a,
move 6 from rhs to lhs by changing its sign
rhs = right hand side
lhs = left hand side
-12 - 6 = - 2a
-18 = -2a
dividing -18 by -2, we get a = 9
Hence, a = 9 is the required answer.