The Pythagorean Theorem is a^2 + b^2 = c^2 where a and b are the sides and c is the hypotenuse.
From the picture you have a = 10 and c = 15.
Now you have:
10^2 + b^2 = 15^2
Simplify:
100 + b^2 = 225
Subtract 100 from both sides:
b^2 = 125
Take the square root of both sides:
b = √125
b = 11.2