P = 2W +2L step is keep 2L in one side of equation , and move every thing else to the other side.
In moving any quantity from one side to other, change the sign. ( equivalent of adding or subtracting equal quantity from both sides of equation).
2L = P - 2W divide both sides by 2:
L = p/2 - w Step is the same as solving X in equation of 8 = 2X + 16 2X = 8 -16 = 8 X =4 . But, here you solve one variable in terms of other variables.