Angle Y is 90 degrees, meaning it's a right triangle. Therefore angle W is the top/tip of a triangle, while X is the angle next to Y. If you plug in the information afterwards and follow the trigonometry rule (SOH CAH TOA) you will find your answer.
When the rectangular sheet rolled along its length, the breadth becomes the height of the cylinder, and the length becomes the circle, so the height is 15 cm
