You'll apply the geometric mean formula. The idea is to multiply the two pieces of the hypotenuse: 5*11 = 55
Then apply the square root to get sqrt(55)
The exact answer in terms of radicals or square roots is sqrt(55)
Using a calculator, the approximate answer is sqrt(55) = 7.416198
If your teacher wants you to use a proportion, then one way to do it is to have
11/y = y/5
11*5 = y*y
55 = y^2
y^2 = 55
y = sqrt(55)
y = 7.416198 approximately