Answer:
[tex]\huge\boxed{\boxed{s=P-t-r}}[/tex]
Step-by-step explanation:
[tex]P=s+t+r\qquad|\text{subtract}\ t\ \text{and}\ r\ \text{from both sides}\\\\P-t-r=s+t+r-t-r\\\\P-t-r=s\Rightarrow\boxed{s=P-t-r}[/tex]
