If someone could explain how to do these problems that would help me a lot thank you.

3. Mathematically show that if d(n) is O(f(n)) and e(n) is O(g(n)),d(n)+e(n) is O(f(n)+g(n)). 4. Consider f(n)=3n2+4n3, mathematically show that f(n) is O(n2),(n2), and (n2)