If the legs of a right triangle adjacent to the right angle are $9$ cm and $12$ cm, what is the length of the hypotenuse?

By the Pythagorean theorem, the hypotenuse is $\sqrt{9^2 + 12^2} = \sqrt{81 + 144} = \sqrt{225} = 15$ cm. This is the familiar $3$-$4$-$5$ triple scaled by $3$.