Depends on whether the "x" is the measure of the hypotenuse or one of the legs of the right triangle.
If is the measure of the hypotenuse, there are two ways to go about it. You can recall that 3-4-5 is a Pythagorean triple and notice that 15 is 3 times 5 and 20 is 4 times 5 so the hypotenuse must be 5 times 5 or 25. Or you can use the Pythagorean theorem to write:
and do the indicated arithmetic.
On the other hand, if is NOT the measure of the hypotenuse, then the hypotenuse must measure 20 and you need to solve:
