Answer:

Explanation:
1) Looking attentively to
. We need two find two numbers whose product is
and their sum is -1.
2) These numbers are -21 and 20 their product is -420 and their sum is -1. Prime factorization might be a good start to find those numbers. Moreover, you can make a list of numbers whose product is -420.
3) Rewrite the trinomial, with those two terms as new b terms.

4) Finally, factor them by grouping them:
