well both of those numbers are divisible by 3. if you divide 3x by 3 you gst x and 33 by 3 you get 11
So 3x + 33 divided by 3 is x + 11. But we can't just throw the three away it has to go somewhere. so we do this:
3 (x + 11)
notice if you distribute the 3 you get 3x + 11
si that is how you factor
