How do you decide what the 'c' and 'n' values are?

1. Let f(n)-1/8 n2 + 1/8. Prove f(n) is: O(n2), 0(n3), (n2), and (n). 2. Let f(n) 4n3-n2 + n. Prove f(n) is: O(n3). O("), (n3), and (1) 3. Let f(n)-3n2 + 4n + 8. Prove f(n) is (n2)