the initial equation is:
[tex]\frac{p}{r}+s=q[/tex]To solve for p we can rest s in bout sides so:
[tex]\begin{gathered} \frac{p}{r}+s-s=q-s \\ \frac{p}{r}=q-s \end{gathered}[/tex]Now we can multiply by r so:
[tex]\begin{gathered} \frac{p\cdot r}{r}=(q-s)\cdot r \\ p=(q-s)\cdot r \end{gathered}[/tex]