Not using calculus if the trajectory is gven by y= 390x -17x(squared) how do you calculate max height

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Y or the height is equal to zero at two points, at X=0 and X=22.94, Y would reach the maximum height half way between these two values of X, that is, X=11.47. 0=390X-17X^2, factoing X(390- 17X), X=0 or X=22.94. The maximum height for this parabolic trajectory is reached at X =22.94/2, that is X=11.47.