ok so
If you are given the hypotenuse (pretend its 4), and you know that it is an isoceles right triangle, you know that the triangle is a 45-45-90 triangle (because you know that the angles = each other since the opposite sides are equal to each other). Since it is a 45-45-90 triangle, the legs will = A, and the hyp would equal to A root 2. you would put the hyp as 4 root 2. To find a you would divide the hyp by root 2. If you do that you can find that A is 4.
Hope this helps!