The x-coord. of the vertex is halfway between the given zeros. Here, it'd be halfway between x=3 and x=9, which comes out to x = 6 (answer).
For a vertical projectile, the equation of motion is expressed h(t)=g+bt+c, where g is gravity, b is the initial upward velocity, and c is the starting height. The maxima of this equation is given by -b/2g, which in this case would be -56/-32, or 1.75 seconds after launch
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