Chapter 6 - Section 6.6 - Rational Equations and Problem Solving - Exercise Set - Page 387: 8

$n=\dfrac{2S-nL}{a}$

Using the properties of equality, in terms of $ a ,$ the given equation, $ S=\dfrac{n(a+L)}{2} ,$ is equivalent to \begin{array}{l} 2S=n(a+L) \\\\ 2S=na+nL \\\\ 2S-nL=na \\\\ \dfrac{2S-nL}{a}=n \\\\ n=\dfrac{2S-nL}{a} .\end{array}