Chapter 1 - Introduction to Algebraic Expressions - 1.3 Fraction Notation - 1.3 Exercise Set - Page 26: 23

$3\times13$

A number is prime if it has only two factors: 1 and itself. We know that the factors of 39 are 1, 3, 13, and 39. 3 is a prime number, so we can divide 39 by 3 $39\div3=13$ 13 is also a prime number Therefore, the prime factorization of 39 is $3\times13$.