Chapter 1 - Some Basic Concepts of Arithmetic and Algebra - 1.2 - Prime and Composite Numbers - Problem Set 1.2 - Page 13: 49

$43\times3$

Since the sum of the digits of $129$ is $12$, which is a multiple of $3$, this number is divisible by $3$. When we divide $129$ by $3$, we get $43$. Since $3$ and $43$ are both prime numbers, the prime factorization of $129$ is $43\times3$