The relationship between the legs and hypotenuse is described by the Pythagorean Theorem, which states that
(leg)² + (other leg)² = (hypotenuse)²
Here, our legs are 6 and 10. Calling our hypotenuse "h", his means that
6² + 10² = h²
36 + 100 = h²
136 = h²
Taking the square root of both sides, we find that h = √136 ≈ 11.66 cm