So we have:
P =2 ( l + w)
We want to get 'l' by itself.
Next we have to divide both sides by 2 , since '( l + w )' is being multiplied by 2
P/2 = l + w
Next you by subtracting both sides by w to get 'l' alone
P/2 - w = l
So the answer is the first one.