Question 121493
start with the common factor 7 __ 7(c^2+2cd+d^2-9f^2)


c^2+2cd+d^2 can be factored into (c+d)(c+d) or (c+d)^2
__ this means we're seeing the difference of two squares


7((c+d)^2-(3f)^2)=7(c+d+3f)(c+d-3f)