In order to factor the expression, you need to find one thing that's a factor of EACH term in the expression. So let's look at the terms.
-- factors of 3x : 1, 3, x, 3x
-- factors of 9 : 1, 3, 9
The only things that are factors of both terms are 1 and 3. We can ignore 1, because 1 is always a factor of anything. That leaves us with 3 . So we'll factor 3 out of the expression.
3x + 9 = 3 (x + 3)
