Answer:
9
Step-by-step explanation:
We can use the Pythagorean Theorem for this because it's a right triangle. We have b and c, we just need a.
So, a^2+12^2=15^2
Which we can make a^2+144=225.
We can then subtract to get a^2=81
To get a by itself, you have to find the square root of 81, which is 9.
a=9
