Okay, the equation is y = -.007x^2 + 0.84 + 0.8 where y is the distance above the water and x is the distance between the supports, both measurements in imperial feet.
If the distance between the supports is 60 ft, then the equation is:

Now since you can't practically have an arch 23.56 feet under the water, I think you forgot an x after the .84 in the original equation.
Using the added x, the equation works out to be:

Your answer should be
26 feet above the water.
Hope I helped :)