This is an isosceles triangle, so both legs are of length 5√2.
Use the Pyth. Thm. to find the length of the hypotenuse. Square 5√2 and double the result: 25(2) = 50; twice 50 is 100. The square of the length of the hyp. is 100, and so the length of the hyp is sqrt(100), or 10 (answer).
°Alternate interior angles are equal °Corresponding angles are equal °Vertically opposite angles are equal °Angles on the same side of tranversal are supplementary