Question 1118286
If your model has to be {{{h=65-9t^2}}}, then the gum hits the ground for h=0;


{{{65-9t^2=0}}}
{{{65=9t^2}}}

{{{t^2=65/9}}}

{{{t=sqrt(65)/3}}}

{{{t=2.687}}}, or to the nearest whole second,  {{{3}}}{{{seconds}}}.


You can find the asked average speeds in the other given time intervals.
{{{(h[2]-h[1])/(t[2]-t[1])}}}