p/q – pq + q/r = rs

Rearrange the formula so that the subject is P

p / q – p q + q / r = r s

Subtract q / r to both sides

p / q – p q = r s - q / r

p ( 1 / q - q ) = r² s / r - q / r

p ( 1 / q - q² / q ) = r² s / r - q / r

p ( 1 - q² ) / q = ( r² s - q ) / r

Divide both sides by ( 1 - q² ) / q

p = [ ( r² s - q ) / r ] / [ ( 1 - q² ) / q ]

p = q ( r² s - q ) / [ r ( 1 - q² ) ]