The cables supporting a straight line suspension bridge are nearly parabolic in shape. Suppose that a suspension bridge is being designed with concrete supports 160 ft. apart and with vertical cables 30 ft. above road level at the midpoint of the bridge and 80 ft. above road level at a point 50 ft. from the midpoint of the bridge. How long are the longest vertical cables?

I got the equation: $\displaystyle {1\over50}(x-80)^2+30=0$ but not sure how I got the 1/50. Can anyone explain?