the perimeter of a square lot is seven times the length of a side, decreased by 12. fine the length of a side.
The perimeter of a square is four times the length of a side, so if we let x be the length of a side you can set up two formulas.
Let's set P to be the perimeter.
So P = 4x (by definition)
and P = 7x - 12
Now you can use these two formulas to calculate x, by setting them to equal each other.