Simple geometry problem

I have a simple problem and after some careful thought I can't yet find the solution:

I'd like to draw the arc and the diagonal line tangent to it.

I only know the values shown, I don't know the angle A. The top of the arc is perpendicular to the vertical line.

I'd like to have sliders for to change each of the three numerical values shown.

Any thoughts?

I'm convinced that this is enough data to construct the geometry, but I am somehow forgetting a geometric identity/property to write the solution.

Thanks in advance.