let

l = length of the rectangle

w = width of the rectangle

A = 56 km^2 in the area of the rectangle

the width of a rectangle is 6 kilometers less than twice its length

w = 2*l - 6

the formula for the area of rectangle is A = w*l so

w*l = 56

by solving the system of equations

w = 2*l - 6

w*l = 56

we find

w = 8 km

l = 7 km

we consider only the positive real numbers since l > 0 and w > 0

the width is 8 km in and the length is 7 km.

if we consider that the length and width should satisfy the inequality l>=w then the problem has no solution (there is no such rectangle).