Answer:
2.25 seconds
Step-by-step explanation
[tex]d=16t^{2}[/tex]
[tex]d=81 feet\\t=?[/tex]
Because you know what distance equals you can plug it into the equation
[tex]81=16t^{2}[/tex]
From there you can solve by dividing the 16 over and then square rooting.
[tex]\frac{81}{16}=t^{2}[/tex]
[tex]5.0625=t^{2}[/tex]
You take the square-root to get rid of [tex]t^{2}[/tex]
[tex]\sqrt{5.0625} =\sqrt{t}^{2}[/tex]
[tex]t=2.25[/tex]
