Step-by-step explanation:
l = length
w = width
l = 3w - 4
the perimeter of a rectangle (the way all around the rectangle) is 2 times the length and 2 times the width.
56 = 2l + 2w
now we are using the first equation and substitute it into the second
56 = 2×(3w - 4) + 2w = 6w - 8 + 2w = 8w - 8
64 = 8w
w = 8 mm
=>
l = 3w - 4 = 3×8 - 4 = 24 - 4 = 20 mm
