6 squared is 36 and 8 squared is 64. Add them together and you get 100, which is 10 squared. Do the same for the other ones and you'll notice that 7 squared and 24 squared added together is 625, which is not 26 squared. In my head 25 squared is 625(check if you don't believe me) so I know that the answer is the third one. The basic formula for a right triangle is a^2 + b^2 = c^2, and that's how you find if three numbers form a right triangle or not. Hope this helps :)
