any number that has 9 as a factor also has 3 as a factor why is this

Let's suppose 9 is a factor of the number N. That means N is 9 times some integer M.

N = 9*M

Since 9 = 3*3, we can also write N as

N = 3*3*M

That means N is 3 times some integer (3*M). So 3 is also a factor of N.