Use a proportion or the Pythagorean Theorem to find y. If needed, estimate your answer to two decimal places.

Answer:
y ≈ 17.32
Step-by-step explanation:
The triangles are all similar, so you have ...
long-side/hypotenuse = y/(5+15) = 15/y
Cross-multiplying gives ...
y^2 = 15·20 = 300 . . . . . . . . take the square root to find y
y = √300 = 10√3 ≈ 17.32