Question

How do I solve P=2L+2W for L

Answer 1

You need to get L by itself. Keep in mind with equality problems that what you do to one side, you must do to the other.
Let me break it down for you.
P=2L+2W, -2w from both sides.
P-2W=2L, now divide the other side by 2.
1/2P-W=L

Answer 2

P=2L+2W
2L=P-2W
L=P/2-(2W/2)
L=1/2P-W