without graphing , whether the given quad function has a maximum or minimum value and then find it's value.

Here's the problem

f(x) = -2x^2+12x

I tried completing the square, but it doesn't work.

-2x^2 + 12x take 12/2+ 6 ^2 + 36 = (-2x^2+12x+36)

Factor out -2(x^2-6x-18) and the problem doesn't factor from here. What am I missing?

Thank You

Jason