Based on given information's, first you find equation of the ellipse. Since a=63 and b=32 equation of the ellipse is x^2/a^2 + y^2/b^2=1 or x^2/63^2+y^2/32^2=1.

Next, you have to find x such that y=14.2. Replacing y=14.2 into equation above and solving for x will give you horizontal distance from center of the arch to where height is 14.2 feet.

Hope this help.