First, make the equation = 0 by subtracting 36 from both sides.
x^2 + 6x + 9 - 36 = 0
x^2 + 6x - 27 = 0
Then you need to know that you want to end up with a format of (x plus/minus a number)(x plus/minus a number) and this assumes that you know how to solve this kind of equation with the FOIL method.
If not, that requires another question and answer!
You need to be quick with times tables and be able to recognize what numbers, when multiplied, will give you 27, but when added or subtracted from each other, will give you 6. When you play around with it, you figure out that 9 * 3 = 27 and 9 - 3 = 6
Then you plug it in so that it makes sense (which ones are subtracted from which)
(x + 9)(x - 3) = 0
From here, you know that if one of the sets in parentheses equals 0, then the whole thing will equal 0, and therefore your two solutions are x = -9 and x = 3