Answer:
√55
Explanation:
Notice that the small triangle in the bottom corner shares an angle with the overall triangle. Also, they are both right triangles. Therefore, they are similar triangles.
Notice that the large triangle at the top shares an angle with the overall triangle and is also a right triangle. Therefore it is also similar to the overall triangle and the smaller triangle.
Writing a proportion between the small and large triangles:
y / 11 = 5 / y
y² = 55
y = √55