Chapter 1 - Section 1.2 - Formulas and Percent - 1.2 Exercises - Page 61: 9

$L=\dfrac{P-2W}{2}$

Using the properties of equality, in terms of $ L ,$ the given formula, $ P=2L+2W ,$ is equivalent to \begin{array}{l}\require{cancel} P-2W=2L \\\\ \dfrac{P-2W}{2}=\dfrac{2L}{2} \\\\ L=\dfrac{P-2W}{2} .\end{array}