Answer
$r=\displaystyle \frac{2V-IR}{2I}$
Work Step by Step
The goal is to isolate the wanted variable on one side of the equation.
First of all, the denominator must not be 0, $r\displaystyle \neq-\frac{R}{2}.$
Multiply both sides with $(R+2r)$
$I(R+2r)=2V$
$ IR+2Ir=2V\qquad$ ... subtract $IR$
$ 2Ir=2V-IR\qquad$ ... divide with $2I$
$r=\displaystyle \frac{2V-IR}{2I}$
