Complete the square for f (x) = x^2-12x+7 what is the value of a for f (x)

I'm not sure what you mean by a.

In a square (x-c) (x-c) = x^2 - 2cx + c^2

-2cx = - 12x, c = 6

(x-6) (x-6) = x^2-12x + 7 + (36-7) = x^2-12x+7 + 29 = x^2 - 12x + 36

I think a is probably either - 6 or 29.