The answer would be 3(x+5).
If you take the GCF (Greatest Common Factor) out of 3 and 15, the greatest common factor is 3. The factors of 3 is 3 and 1 while the factors of 15 is 1 , 3, 5, and 15. The highest one that matches up is 3, so you would divide the equation by 3 and then put it on the other side.
3x+15
*divided by 3*
3(x+5)
