A cross-section of an irrigation canal is a parabola. If the surface of the water is 33ft wide and the canal is 15ft deep at the center, how deep is it 4ft from the edge?

I assume the following is how it's done. Again, looking more towards confirmation and possible corrections if incorrect. Thanks.

Equation is y=ax^2

y=15

x= 16.5

15 = a(16.5)^2

a= 0.055

So, to get depth at 4 ft from edge, I assume my new x value is 12.5.

Derived from 16.5 - 4 = 12.5

So:

y= 0.055 * (12.5)^2

y = 8.59

To get depth, I assume subtract from center depth of 15ft.

15ft - 8.59ft = 6.41ft

Answer is 6.41ft.

Correct? Thanks.