Relationship between Area of Square and Rectangle

(a) A rectangle has the same area as a square. The long side of the rectangle is four times the length of the short side. The perimeter of the square has length 160 mm. How much longer is the perimeter of the rectangle than the perimeter of the square?

(b) Same question but this time the perimeter of the square is L mm and the long side of the rectangle is x times as long as the short side.

I've already gotten (a) correct, but no matter how many times I try (b) I haven't gotten the right answer.