My sons school does not have text books. I have been out of school many years and don't remember how to do factoring. If I could see some examples maybe I would remember and be able to help him.

Assuming the numbers after the letters are exponents.

first you find what Is in common with the two numbers. Since in the first set you have a 21 and a 7 you can pull 7 out and leave 3 on the left since 3*7 is 21. On the right 1 would be left.

Then you pull out b squared since you have that inside both sides

That makes the answer 7b^2(b-1)

Using the technique I just told you the answer for the second side is 4(3b-1)