The vertex of the parabola occurs at x = 3 (do you see why?). So the maximum height occurs at x = 3:
In a children's play park a ball is kicked so that its flight path can be modelled by the equation h= -ax(x-6) , where h is the height of the ball when it is x metres from the point where it is kicked. If the maximum height of the ball is 2m, what is the value of a?