Undergrad Understanding the Coordinates in the Lagrangian for a Pendulum

[p1ndol]

So I've been studying classical mechanics and have come across a small doubt with the solution provided to the problem in question from Landau's book. My question is: why are the coordinates for the particle given as they are in the solution? I imagine it has something to do with the harmonic oscillator, but I'd like to properly understand. I appreciate any kind of help, and I'm sorry if this post is somehow incorrect, it is my first one regarding questions.

 

[ergospherical]

Did you look at the figure? For instance in (a), the support point $p$ has coordinates $\mathbf{r}_p = a(\cos{\gamma t}, -\sin{\gamma t})$ and the radius vector from $p$ to $m$ has coordinates $\mathbf{R} = l(\sin{\phi}, \cos{\phi})$ then the coordinates of $m$ are nothing but those of the vector $\mathbf{r}_p + \mathbf{R}$.

 

[p1ndol]

I understand it now, thanks!