Because it's a right triangle and you are looking at the lengths of the sides, you want to use the Pythagorean theorem: a^2 + b^2 = c^2
The key to using this is that c MUST be the length of the longest side, in this case that's x.
4^2 + 5.5^2 = x^2
16 + 30.25 = x^2
46.25 = x^2
Then take the square root of both sides:
± 6.80 ≈ x
Technically the equation has two solutions, but only the positive one makes sense here.