Show that the area ,A,of the rectangle PQRS is given by $2p(16-p^2)$ Show that the area ,A,of the rectangle PQRS is given by $2p(16-p^2)$

For question 10a) I am having trouble finding the $p^2$
I understand the 2p and is able to find the 16 using dy dx=0
Please help
(https://i.stack.imgur.com/GemEv.jpg)

Let $S(4-p,0)$, then $P(4-p,(4-p)(4+p))$ or $P(4-p,16-p^2)$. Then you have $A=SR\times SP$.
