Since the sides are marked with the little lines, you know they are congruent with each other. Since the sides are congruent, you know the hypotenuse will be congruent.
You can argue that these triangles are similar, but the difference is that similar triangles are the same shape, but not the same size. These triangles are congruent because they are the same size and shape. Therefore, the hypotenuse is congruent.
Corresponding angles are those which are in the same relative place between the cross-line and the parallel axes. You will see that the (corresponding) angles are identical if and only if the lines are parallel.
