We can use the formula for a parabola: 
where
is the origin of the parabola and 
is some constant scaling factor.
We are given that the center of the arch on the bridge is
. This is our origin. So our equation is:
Now we just need to find c. We can do this by plugging in one of the other points given:
So our final equation for the arch of the bridge is:
We are given that the center of the arch on the bridge is
Now we just need to find c. We can do this by plugging in one of the other points given:
So our final equation for the arch of the bridge is: