I am asked to show that for a hyperbolic quadrilateral with $3$ right angles and one point on the absolute with finite sides of length $a,b$ then
$$\sinh(a)\sinh(b)=1$$
I have constructed a line so that I can use the angle of parallelism formula but I'm a bit stuck at this point.
I'd really appreciate any help!
Edit: I have the complimentary angles of parallelism as suggested below but I'm still a little stuck on how to apply these without long caluculations which I can't seem to get to work out.