For this problem you have to derive the equation given:
[tex]\begin{gathered} y^{\prime}=(-18x^2+36x)^{\prime}\text{ = -36x+}36 \\ \text{Now we equal to zero} \\ -36x+36=0 \\ \text{and we solve for x} \\ x=1 \end{gathered}[/tex]If we derive y again we notice that y''=-36 that means that a maximum is reached for x=1, then the maximum height of the ball is
[tex]y=-18(1^2)+36(1)=18[/tex]y= 18 feet
