This video is about using the grouping factoring method. We're using a four term polynomial. There is a even number of terms. We can break them into two groups of 2. The first group of "2" the common factor is "a." The last group, the common factor is "-2b". The reason we use -2b is to assure that what is in the first group is the same as the second group. Because of the distributive property of multiplication over addition, we take the a-2b and put it in one bracket. Multiply by 3x+1. With 6 terms, set in 2 groups of 3. Factor out "x". x(9-2b-3) y(a-2b-3). It's the same content in both. (x+y) (a-2b-3). Multiply these two by each other to get the original question.
The viewer learns to do grouping factoring method of polynomials.