Trying to work out the answer to the following:

Let f1 and f2 be two functions from N to N such that:

f1(x) = x^2 and f2(x) = 3x + 6x^2.

Question 1) What is the function f1 ∘ f2?

Question 2) What is the function f2 ∘ f1?

I am very confused by function composition and any help would be greatly appreciated!

Thanks! :D