The width of the rectangle is twice its length.
So lets assume the width to be------ 2x feet
Then the length will be ------ x feet
The perimeter is given as ---- 78 feet
The formula for perimeter of a rectangle is ; 2[ l+w ] where l is length and w is width
Use the assumed measurements in the equation for perimeter as;
2[ l+w ] =78
2{x +2x] =78
2x +4x =78
6x = 78
x=13 feet
Length = x = 13 feet
Width = 2x = 2*13 = 26 feet