Since a stop sign is an octagon it has eight sides. So the perimeter will be:
p=8x, p=perimeter and x=side length. We are told that the perimeter is equal to:
16x^2-80x so we can say:
16x^2-80x=8x subtract 8x from both sides
16x^2-88x=0 factor out 8x
8x(2x-11), since we know x>0
x=11/2
x=5.5 units