(h,k) is the vertex of the parabola.Originally Posted byusm_67

Here, the parabola is vertical and opens downward, so the vertex is the maximum height.

(h,k) = (0,10) -------if the origin (0,0) of the x,y axes is at water level and vertically below the max height.

y = a(x-h)^2 +k ---------(i)

y = a(x-0)^2 +10

y = a(x^2) +10 --------(ii)

At righthand end of the parabola,

x = 30/2 = 15

y = 0

Substitute those into (ii),

0 = a(15^2) +10

0 = 225a +10

-225a = 10

a = -10/225 = -2/45 = -1/22.5

Therefore, if the origin (0,0) of the x,y axes is at water level and vertically below the max height, the parabola is

y = (-2/45)x^2 +10. -------answer.