The length of a rectangle is twice its width. The perimeter of the rectangle is 126 feet

I'm assuming the question is to find the length and width.

first to find the width:

126:6 = 21 the width is 21 feet

(I devide by 6 since the width of the rectangle is half the length and the perimeter of a rectangle is 2 widths plus 2 lenghts that ads up to 6 widths)

next I find the length:

21x2 = 42 the length is 42 feet

(I multiply the answer of the width by 2 since the length is twice the width)