Start with
[tex]S = \pi r^2 + \pi r l[/tex]
Subtract [tex]\pi r^2[/tex] from both sides:
[tex]S - \pi r^2=\pi r l[/tex]
Divide both sides by [tex]\pi r[/tex]
[tex]\dfrac{S-\pi r^2}{\pi r}=l[/tex]
And thus we solved the expression for l:
[tex]l=\dfrac{S-\pi r^2}{\pi r}[/tex]
