To start with, the hypotenuse for the triangle with in it is the same as the side opposite angle in the upper triangle. Call this side and we have

As for the third side of the triangle with in it, call this side . If you complete the rectangle, you'll have another right angle triangle with that side opposite and with a hypotenuse of . That means

So finally, in the triangle with in it, we have by Pythagoras

I think that's the simplest you can get it