Let x represent the height of the person and y the weight.
y - 180 = (80 - 180)/(4 - 6) (x - 6)
y - 180 = -100/-2 (x - 6) = 50(x - 6) = 50x - 300
y = 50x - 300 + 180 = 50x - 120
y = 50x - 120
A person 5.5 feet tall will weigh 50(5.5) - 120 = 275 - 120 = 155 pounds.