The formula for the perimeter of a rectangle is p=2(l+w).

solve for l.

answer choices are in the picture

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.

Are you asking for 6 or 15?