If you want to solve this for P you need to get P on one side of the equation by itself. If I am understanding the way this question is written you need to multiply both sides by 2 first
That would give you 2L=P-2W
Then you can add 2W to both sides and you are left with:
2L+2W=P
From the looks of this it may be referring to a rectangle. L = Length, W = Width and P = perimeter?