You can find this by using the pythagorean theorem.
The Pythagorean Theorem states that the squares of the legs add up to the square of the hypotenuse.
The hypotenuse is the longest side, which is opposite the right angle.
The other 2 sides are the legs.
Here, the hypotenuse is BC which is 17.
AC is one leg. We are trying to find the other leg.
So you set up the equation AB^2+ 15^2 = 17^2.
This simplifies to AB^2 + 225 = 289.
AB^2 = 64.
AB = 8.