Max/ min problem please please help I just don't get this part of the question (1 Viewer)

[kpad5991]

Max/min problem
The diagram in the link below represents in metres the dimensions of a small garden.
Show that y= (20-x^2).

https://imgur.com/gallery/7R1PA

 

[jazz519]

https://drive.google.com/open?id=1eLT0Fudr8clW3WBrrNiFYP_lJ1i9s5fn