So I have finally gotten the hang of the ac method, but I have some trouble completing my quiz.

How do I factor something that doesn't have a GCF, like

x^2 + 4xy - 21y^2

And also, I don't get the Reverse Foil method.

By factoring 10z^2 - 9z - 9 this way, I'm supposed to get (5x+3)(2x-3), but I don't see how.

And how do you get (x-7)(x+6) from x^2 - x - 42 ?

Help, please. Thank you.