what are the prime factors of 72

First, divide by 2:  72 = 2 x 36

Since 36 is even, divide again by 2:  72 = 2 x 2 x 18

Since 18 is even, divide agian by 2:  72 = 2 x 2 x 2 x 9

Since 9 = 3 x 3, finish:  72 = 2 x 2 x 2 x 3 x 3

So, the prime factors are 2 and 3; three 2s and two 3s.