Question

How do you factor by grouping `3x^3 + 15x^2 - 4x - 20`?

Answer 1

Collect`x` betwen the first and third term and `5` between the second and fourth:
`x(3x^2-4)+5(3x^2-4)=`

collect #(3x^2-4);

`=(3x^2-4)(x+5)`