Question

hypotenuse of an isosceles triangle is c. find the area of its triangle

Answer 1

This is the picture that your problem suggests, right?

Answer 2

You can use trig functions to solve this pretty quickly. Because it's an isosceles right triangle, you know that the other 2 angles in the triangle have to 45 degrees each. Thus,
\[ \sin(45) = \frac{sideLength}{c}\]

Does that make sense?