Data Structures, Algorithms, & Applications in C++
Chapter 3, Exercise 3

(a)

2n + 7 is asymptotically bigger than 1. Therefore, 2n + 7 != O(1).

(c)

5n3 + 6n2 is asymptotically bigger than n2. Therefore, 5n3 + 6n2 != O(n2).