how do you factor?

That is the difference of squares, so use the pattern

a² - b² = (a + b) (a - b)

(Once you have factored, you can remultiply to make sure the factors really give 49x² - 9 as their product.)

Since this is probably a homework problem, I will follow KA guidelines and not just give you the answer, but I'd be happy to help you step through it or to point you to the KA videos/articles about the difference of squares pattern.

~ Janice, KA user