Find the least integer n such that f(x) is O(xn) for each of these functions

a) f(x) = 2x3 + x3 log(x)

b) f(x) = 3x3 + (log(x))4

c) f(x) = (x5 + x2 +1) / (x3 +1)

d) f(x) = (x4 + 5log(x))/(x4 +1)