A drainage ditch has a parabolic cross-section.

If the ditch is 15 ft deep at its center, and 60 ft wide across the top, what is the depth of the ditch at a distance of 5 ft from either side?

My current thought process is having a vertex point at (0,-15), being the bottom of the ditch. Then creating an equation for the parabola, which ended up being y = 1/60x^2 - 15, then plugging in 25 to get 4.58 feet, is this correct?