Actually, this is not about angles. It's about the length of the sides in a right triangle.
In EVERY right triangle, the squares of the lengths of the short sides add up to the square of the length of the longest side. You're in high school math,so I'm SURE you've heard that in class before ... possibly even just before you were assigned this problem.
Let's say that again: The squares of the lengths of the sides that meet at the right angle add up to the square of the length of the longest side. In the triangle in this particular problem, that means a² + b² = c²
You know the lengths of 'b' and 'c', so you shouldn't have any trouble finding the length of 'a'.
