The perimeter of a stop sign has a perimeter of 16x2 - 80x. what is the length of one side?

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