The cable of a uniformly loaded suspension bridge hangs in the form of a parabola. The roadway which is horizontal and 100 m long is supported by vertical wires attached to the cable, the longest wire being 30 m and the shortest being 6 m. Find the length of a supporting wire attached to the roadway 18 m from the middle.

Question

$\begin {array} {1 1} (A)\;9.11 \: m & \quad (B)\;9\: m \\ (C)\;-9.11 \: m & \quad (D)\;-9\: m \end {array}$