f(x) = x^2 - 12x 36

find a value of x such that f(x)= 9

First, I assume that what you meant to write is:
f(x) = x^2 -12x +36

Please confirm.

You know this equals 9, so:


9 = x^2 -12 x +36


Subtract 9 from both sides:


0 = x^2 -12x +27


Now you can factor this and find two solutions for x. Post back with what you get for answers and we'll check it for you.