For this case we can solve the problem by using the Pythagorean theorem.
We know:
1) Length of the ladder representing the hypotenuse
2) Length from the wall to the base of the ladder, which represents the base of the triangle.
We want to know:
1) The height of the triangle that represents the height of the wall that the ladder reaches.
We have then:
Answer:
The ladder reaches a height of 15 feet.