What is the factor tree for 1521?

Since the number is NOT even , then it will not divide by '2'

So try the next [ri,e number. which is '3'

Hence

3)1521 = 507

3)507 = 169

169 will not divde by '3,5, 7,11, but it will divide by '13' the next Prime number.

13)169 = 13

13. 13 = 1

So reduced to its factors it is is 3,3,13,13.