Difference of Squares

If a polynomial can be written as a² – b², then it can be factored as a difference of squares:

a² – b² = (a + b)(a – b)

This is because, when you use FOIL to expand the right side, the ab terms cancel out:

(a + b)(a – b) = a² – ab + ab – b²

This is a difference of squares with a = 3p and b = 7q.

9p² – 49q² = (3p – 7q)(3p + 7q)

You should verify this using FOIL.