If we have:
[tex]p=2w+2l[/tex]
To solve for l we can start by inverting the sides and substracting 2w from both sides so that the term with l becomes alone in the left side:
[tex]\begin{gathered} p=2w+2l \\ 2w+2l=p \\ 2w-2w+2l=p-2w \\ 2l=p-2w \end{gathered}[/tex]
Now, we can divide both sides by 2 so thay the 2 in 2l gets canceled:
[tex]\begin{gathered} 2l=p-2w \\ \frac{2l}{2}=\frac{p-2w}{2} \\ l=\frac{p-2w}{2} \end{gathered}[/tex]
So, the answer we would get is
[tex]\frac{p-2w}{2}[/tex]
