Pre-Calculus Parabola Problem that's driving me insane

Question: An experimental model for a suspension bridge is built in the shape of a parabolic arch. In one section, cable runs from the top of one tower down to the roadway, just touching it there, and up again to the top of a second tower.

The towers are both 6.25 inches tall and stand 50 inches apart. At some point along the road from the the lowest point of the cable, the cable is 0.56 inches above the roadway.

Find the distance between that point and the nearest base of the tower.

My Work:

The two points should be (-25, 6.25) and (25, 6.25).

The equation used to solve is x^2 = 4ay.

Plug in x and y to find a, which equals 25.

Equation is now x^2 = 100(y)

If the distance above the roadway is (0.56), then x^2 = 100(0.56).

x^2 = 56

x = about 7.4

So I got 7.4 as the answer - I went to the back page for help, apparently the answer is 17.5 - but I have no clue how that's possible, and how I'm so way off!

Please help, I have my final tomorrow - bonus points if you include a picture (but it's not necessary)!