The answer has to look like (a ? 3b)(a ? 7b).
We cannot have both + or both - for ? because
of the -21b²; one ? has to be a + and the other
a -. The question is which one. The clue is in
the +4ab. This determines that the 2nd brackets
should have the +; so the 1st must have the -. So
a²+4ab-21b²=(a-3b)(a+7b).
We cannot have both + or both - for ? because
of the -21b²; one ? has to be a + and the other
a -. The question is which one. The clue is in
the +4ab. This determines that the 2nd brackets
should have the +; so the 1st must have the -. So
a²+4ab-21b²=(a-3b)(a+7b).