Hey could anyone assist me with this problem?

Find the least integer $\displaystyle n$ such that $\displaystyle f(x)$ is $\displaystyle O(x^n)$ for each of these functions.

a) $\displaystyle f(x) = 2x^2+x^3logx$

b) $\displaystyle f(x) = 3x^5+(logx)^4$

c) $\displaystyle f(x) = (x^4+x^2+1)/(x^4+1)$

d) $\displaystyle f(x) = (x^3+5logx)/(x^4+1)$