The idea is to try and break those radicands up into 2 numbers multiplied together with one of them being a perfect square. When you do that, this is what you get: [tex]7 \sqrt{4*7}-5 \sqrt{9*7} [/tex]. In the first radicand, we will pull out the 4 as a 2, and in the second one, we will pull out a 9 as a 3. Then we have [tex]14 \sqrt{7} -15 \sqrt{7} [/tex] which gives you [tex]- \sqrt{7} [/tex], choice A from above.
