The hypotenuse can always be found with the Pythagorean Theorem: (a^2)+(b^2)=(c^2).
Essentially, this says the length of the legs are squared then added. That sum will be equal to the the longest side (hypotenuse) squared.
So, your first step should be to identify the legs and the hypotenuse. Label the legs "a" and "b", and the hypotenuse should be labeled "c". From there, you can plug in the values to your theorem and solve for the remaining unknown side.