Imagine a quadrilateral $ABCD$ where $AB$ is ∥ to $CD$ and ∠D is 2 times as large than ∠B.
We will add in 2 more variables, $a$ and $b$, where $a$ is the length of $AD$ and $b$ is the length of $CD$.
Our goal is to find the length of $AB$ in terms of $a$ and $b$
This is already very hard to visualise so I will be very happy if someone explains this to me, thanks!
First note that the corners of $ABCD$ rotate round it: the sides are $AB$, $BC$, $CD$ and $DA$. Hence if you know that the angle at $B$ is half the angle at $D$, you can start by drawing a sketch where you start by drawing $AB$, then put $B$ at, say, $60$ degrees to draw $BC$ (don't make $BC$ all that long, by the way), then you draw $CD$ parallel to $AB$ but stop when $D$ is twice $B$ and then join $D$ to $A$.
You get a trapezoidal shape.
When you have drawn it, you will visualize it just fine.