Use the Pythagorean Theorem: [tex]a^2+b^2=c^2[/tex] where a and b are the legs of the right triangle, and c is the hypotenuse (longest side!).
There is no picture provided with the problem, so it might be possible for the right angle to be at C and the hypotenuse is AB. But that won't work with the answer choices given, because then [tex](AB)^2=7^2+8^2 \\ (AB)^2=49+64=113 \\ AB = \sqrt{113}[/tex]
Now you know that the 7 must be a leg and 8 the hypotenuse. Then [tex]7^2+(AB)^2=8^2 \\ 49+(AB)^2=64 \\(AB)^2=64-49 \\ AB=\sqrt{15}[/tex].
By the way, it cannot be that 7 is the hypotenuse because it's the longest side and 8 is longer.
