(3p - 7)(3p + 7) has the form of (a-b)(a+b). So (a-b)(a+b)= a^2-b^2. So in (3p - 7)(3p + 7), a=3p and b=7. So (3p - 7)(3p + 7) = 3p^2 + 7^2 = 9p^2-49. So (3p-7)(3p+7)= 9p^2 - 49.
You can check your answer (very important): We'll use p=1. (3-7)(3+7) = -40 9-49 = -40 The answer has been approved.