Finding the angle between a wall and a broken rope

[bolzano95]

Homework Statement

The following problem is to find an angle α

Relevant Equations

α=?

I tried to work out the solution using h. I get a trapezium, but there is always a lack of additional information. I looked up the solution and BAM, there is this simple equation for sinα.
I really don't understand how do we get such a solution. I leaned you can use trigonometric functions in a right triangle, but here I really don't get it how to work out α.

 

[Orodruin]

Make a straight line continuing the right rope to the left until it hits the wall. This continuation has the same length as the left part of the rope. This line and the right part of the rope together therefore has the same length as the entire rope. Use trigonometry with this as the hypothenuse.