Case 1:
  f(x) = x+1
  g(x) = 4
  f(g(x)) = 5

Case 2:
  f(x) = 2x^2-x+5
  g(x) = -3x^3+x
  f(g(x)) = 18x^6-12x^4+3x^3+2x^2-x+5