To do this you look for common factors in the two terms. since 96 = 4 * 24... you can factor out a 24... and the b is common to both so you can factor that out too... which means you are factoring out 24b and dividing each term by 24b to find out what remains in the parentheses. Thus:
24b (24b2/24b + 96b/24b) = 24b(b+4)
The key point is to remember that when you factor, you divide each term by what you are factoring out.