Question 555571
Let Length be "x"

Then width = 2x-6

Area = x*(2x-6) which is equal to 56 Sq Km

so the eqn is
x*(2x-6)=56

>2x^2 - 6x = 56

> 2x^2 - 6x - 56=0

> x^2 - 3x-28=0 (dividing by 2)

> x^2 -7x + 4x -28 = 0

> x(x-7) +4(x-7) = 0

> (x+4) (x-7) = 0

So x = -4, 7

As length can't be negative, so the answer is 

Length =7km, Breadth = 8km