A formula like P = 2L + 2W is not written in stone — it is one arrangement of a relationship, and you are allowed to rewrite it so that any letter you like stands alone. The method never changes: look at the variable you want, notice what has been done to it, and undo those things from the outside in. Nothing here is guesswork. Every step is one honest move applied to both sides.
What is wrapped around it?
the variable you want a wrapper still on it a wrapper you undid
Check it with numbers
Set the quantities you would actually know. The rearranged formula computes the missing one — then we substitute it back into the original formula to prove it really works.
Pick a formulathen choose which letter to solve for