Of all the rectangles with a perimeter of 168 feet, find the dimension of the one with the largest area

The rectangle with the largest area with a given perimeter will be a square - so the sides will be equal. So we need to find length of side, L, such that 4*L=168. L = 168/4 L=42. So the dimensions of the rectangle that maximizes the area with a perimiter of 168 feet are: 42 feet by 24 feet.

Iknow one of the answers is a and maybe b but not 100% sure about b