We know, perimeter of the rectangle = 2(l +b)
where, l = length and b = breadth
Here, the perimeter of the rectangle is 48 kilometers and the height is of 10 kilometers. We can take height as the breadth.
=> Perimeter = 2(l + b)
=> 48 = 2(l + 10)
=> 48/2 = l + 10
=> l + 10 = 24
=> l = 24 - 10
=> l = 14 kilometers