It has maximum height at horizontal distance 9, so, h = 9 Put this in eqn,
Now, since it starts off at initial height 2, so, point (0, 2) lies in eqn (1)
Also, birdie strikes the floor at horizontal distance 20, so, point (20, 0) lies on eqn (1) also.
Plug in these point in eqn (1) and solve to find the values of a and k and put these values in eqn (1), you will get the equation
Please see attached graph for clarification.
Did you get it?