Chapter 1, Equations and Inequalities - 1.4 Rewrite Equations and Formulas - 1.4 Exercises - Problem Solving - Page 32: 36a

$r=\dfrac{(P-2x)}{2 \pi}$

As we are given that, $P=2\pi r+2x$ Subtract $2x$ from each side. The, we have $P-2x=2 \pi r$ Now, divide each side by $2\pi$. The formula for $r$ can be defined as: $r=\dfrac{(P-2x)}{2 \pi}$