From the right triangle with hypotenuse 25 and base 15, we know the height of the trapezoid is 20, by the Pythagorean Theorem.
Another right triangle is with hypotenuse 52, height 20, and base x+15.
So (x+15)^2+400=52^2
This simplifies to x^2+30x-2079=0
Factorizing, we get (x-33)(x+63).
Since x=-63 makes the area negative, x=33.
Plug x into your area equation to get your answer.