We want to show that $△\mathrm{ART}=△\mathrm{PEN}$ and we have to use SSS criterion. We have  $\mathrm{AR}=\mathrm{PE}$ and $\mathrm{RT}=\mathrm{EN}$. What more we need to show?

For SSS property all the three sides of $△\mathrm{ART}$ must be equal to all the three sides of $△\mathrm{PEN}$.

Thus, 

$\mathrm{AR}=\mathrm{PE}$

$\mathrm{RT}=\mathrm{EN}$

And we also need to show $\mathrm{AT}=\mathrm{PN}$.