We assume the right angle is at A. Then the Pythagorean theorem tells you
... BC² = AC² + AB²
... 17² = 15² + AB²
... 289 - 225 = AB² . . . . . . subtract 15²
... 64 = AB² . . . . . . . . . . . . simplify
... 8 = AB . . . . . . . . . . . . . . .square root
The length of AB is 8.
