Explanation:
the area of a rectangle is
length × width
the perimeter of a rectangle is
2×length + 2×width
we know the area of this rectangle :
128 = length × width
and we know that the length is twice the width :
128 = 2×width × width = 2×width²
64 = width²
width = 8 cm
therefore, length = 2×8 = 16 cm
the perimeter is then
2×16 + 2×8 = 32 + 16 = 48 cm