## Elementary Algebra

$11$
Using the rule $(a+b)(a-b)=a^{2}-b^{2}$, we obtain: $(3\sqrt 2-\sqrt 7)(3\sqrt 2+\sqrt 7)$ =$(3\sqrt 2)^{2}-(\sqrt 7)^{2}$ =$(3)^{2}(\sqrt 2)^{2}-(\sqrt 7)^{2}$ =$(9)(2)-(7)$ =$18-7$ =$11$