In the image, you can see what information is given. The angle $\varphi$ is given as well. It just isn't assigned any value, but $\varphi < 90^\circ$. The goal is to find the angle $\theta$.
I'm not sure if it's even possible to find the angle from this information, but it seems like it's possible. You can easily calculate $h_3$, but what you need to find is the length of $BE$, so that you can find one of the catheti of the right triangle where $\theta$ is. Note that $AD$ is not a line ($AC$ and $ED$ are not on the same height). For the length $BE$, I thought that you could find the angle at $B$ of the triangle $\triangle EBD$, but I'm not sure how and even if it's possible.
I would appreciate if someone had an idea for how to find angle $\theta$.
It's not possible. You can imagine turning D into a hinge and varying the angle CBD by moving D down and to the left, which changes $\theta$.