First we need to differentiate y
[tex]\\ \sf\Rrightarrow \dfrac{dy}{dx}[/tex]
[tex]\\ \sf\Rrightarrow \dfrac{d}{dx}(-16x^2-110x+15)[/tex]
[tex]\\ \sf\Rrightarrow -32x-110[/tex]
Now let it be 0
[tex]\\ \sf\Rrightarrow -32x-110=0[/tex]
[tex]\\ \sf\Rrightarrow -32x=110[/tex]
[tex]\\ \sf\Rrightarrow x=|110/-32[/tex]
[tex]\\ \sf\Rrightarrow x=3.44s[/tex]
Put it in y
[tex]\\ \sf\Rrightarrow y=15-110(3.44)-16(3.44)^2[/tex]
[tex]\\ \sf\Rrightarrow y=204.05ft[/tex]