This question involves the Pythagorean Theorem, which is a²+b²=c², which basically means, one side of the right angle added to the other side of the ride angle is equal to the hypotenuse/c/the longest line. In order to find a, there are two methods.
1) You split the triangle into 2, and find the middle. Which in this case, would be √15²-9² which is equal to 12. We can then add 12² to 16² to get a² which is 20.
2) Now the other way is to take it as an entire triangle. The hypotenuse is 9+16 which is 25, and √25²-15² will give us a, once again. Therefore, a = 20.
In both methods, the answer to a is 20.