



 
Hi Sal, I drew a rough diagram of the situation you described.
S is the tower at the south end of the bridge, N is the tower at the north end and M is the middle of the bridge. I want a coordinate system on the diagram with the surface of the bridge the Xaxis with positive to the right and the Y axis vertical with positive upwards. The question is where to put the origin. The algebraic description you have is \[ h(x)=a(x30.5)^2+1.5.\] The distance from S to N is 61 metres and half of 61 is 30.5. In the algebraic expression when $x = 30.5$ you get $h(30.5) = 1.5$ metres and you know that the height of the cable is 1.5 metres at the midpoint M. Hence M is 30.5 metres from the origin and hence the origin is at S. You know that the height of the cable above the point S is 15.25 metres but this height is $h(0).$ Solve for a. Penny  


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