If there is a right triangle, you can Always use the Theorem of Pythagoras. As two of the sides of the triangle have the Same length, the right angle has to be between them(if you dont know why: try to Draw a right triangle with to sides that have the Same length where the right angle is Not between them! It doesnt Work!)
c^2=68^2+68^2
c=√(68^2+68^2)