You should have no trouble doing this one by the time you're in high school.
A = 2(L + W)
Divide each side by 2 : A / 2 = L + W
Subtract 'L' from each side: A/2 - L = W
By the way ...
If 'L' and 'W' in this equation are the length and width of a rectangle,
then 'A' is its perimeter.
