√5
Because the triangle is a right triangle, we are able to use the Pythagorean Theorem to find the final side. Keep in mind that we have the hypotenuse and a leg, so we don't need to solve for the hypotenuse, c.
a² + b² = c² Pythagorean Theorem
√10² + b² = √15² Substitute
10 + b² = 15 Exponents
b² = 5 Subtract 10 from both sides
b = √5
The missing side is √5.
Hope this helps!