I need a step by step explanation of finding the GCF of (6ysquare -3y)(y+7)

The greatest common factor, or GCF, is the greatest factor that divides two numbers. ... If there are no common factors, the GCF is 1.

That is the situation on your case.
6y^2 -3y and y+7 do not have common factors.

1 answer

(6ysquare -3y)(y+7)
= (3y) (2y - 1) (y+7)