Answer
$L=\frac{2s-na}{n}$
Work Step by Step
We are given that $s=\frac{n}{2}(a+L)$. We can use the distributive property to multiply out the terms on the right side.
$s=\frac{na}{2}+\frac{nL}{2}$
Multiply all terms by 2 in order to eliminate fractions.
$s\times2=\frac{na}{2}\times2+\frac{nL}{2}\times2$
$2s=na+nL$
Subtract na from both sides
$2s-na=nL$
Divide both sides by n.
$L=\frac{2s-na}{n}$
