You can put this solution on YOUR website! (g o f)(x) simply means g(f(x)). Since you know f(x), you can plug that into g(x) to find the composition.
f(x) = x - 36 (hard to tell from your text; i assume this is what you meant)
g(x) = 6x + 3
g(f(x)) = 6(x - 36) + 3
g(f(x)) = 6x - 216 + 3
g(f(x)) = 6x - 213
(g o f)(x) = 6x - 213