Making r the subject regards parallel resister circuits

Making r the subject in a parallel resister circuit

Are you talking about Ohm's law?

V=IR

therefore

R=V/I

If you have n resistors in parallel, you have this relation:

\Large R_{effective} = \frac {1}{ \frac 1R_1 + \frac 1R_2+ ... + \frac 1R_n}