So, I have a linear equation, and I need to solve for some variables in said equation. However, since I don't know much about matrices, I don't know how to solve for the variables. The equation in question is the one from the Wikipedia page on perspective projection. It is as follows:
I know $a$, $c$, and $d$, but I need to find $\theta$. Let's say that $a$ is (5,0,0), $c$ is (10,0,0), and $d$ is (10,5,0). How, then, would I solve for $\theta$?