I have come accross a question which i cant get my head around, but i am guessing it is to do with groth and big o notation?

Could some on point me in the right direction for finding some online material to learn this type of maths? as im lost with it!

Consider the functions

"f1(x) = x3 +3x2 +2x+1 and f2(x) = x4 +12x3 +7x2 +2x+1.

What is the smallest value of m in N such that

f1(x)+ f2(x) ∈ O(xm)?

Justify your answer briefly."

Many Thanks