Question

How do you factor completely `a(a+1)(a+2) - 3a(a+1)`?

Answer 1

(a-1)a(a+1)

Explanation:

We note that `a(a+1)` is common to both parts of the expression, i.e.
`a(a+1)[(a+2) - 3] =a(a+1)(a-1)`
Or if you prefer, you can have them sorted:
`(a-1)a(a+1)`