



 
Taranjeet, I'm not sure I understand either but I'll show you what I think is the situation. First I think there is a typo in what you sent. I think the function should be
This is a parabola that opens downward and crosses the xaxis at x = 0 metres and x = 120 metres. This gives the height of the arch above the river so the endpoints of the arch are on the xaxis. The road is 50 metres above the river so the image I have is
If this is the correct interpretation then you can find the xcoordinates of the points P and Q. These are the values of x for which
I don't know if this is where the vertical supports are placed. I hope this helps,  


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