The triangles are congruent right triangles.
The left triangle also has a hypotenuse measuring 10.4.
Use the Pythagoras theorem.
x^2 + 5.5^2 = 10.4^2
x^2 = 108.16 - 30.25
x^2 = 77.91
x = 8.82666...
Answer: x = 8.8
