How do you find the length of hypotenuse when the right triangle has an altitude?

All triangles have an altitude. In fact they all have three of them. Whether or not they have an altitude, the important thing when trying to determine the length of the hypotenuse is what information you have on the lengths of the sides.

Altitudes, medians can help determine the lengths of sides, as can angles. You need a minimum of 3 pieces of information. There is only one in the question: the fact that the triangle has a right angle.