You can put this solution on YOUR website!
Recall, given some (a^2 -b^2), it always factors into (a+b)(a-b).
Well this is exactly what you have under your nose. (9-c^2d^2). Would it help if we simplified this to (9-(cd)^2)? Since (cd)^2 = c^2 d^2. Now it is apparent we have two perfect squares. So we can factor it in to
Let me know if this is clear!
Hope this helped.