The period of the pendulum is 11.11 seconds.
EXPLANATION
From the given equation,
L= 0.81t² -----------------------------------------(1)
But L= 100 feet
Substitute the value of L into equation (1)
That is;
100 = 0.81t²
Divide both-side of the equation by 0.81
[tex]\frac{\cancel{0.81}t^2}{\cancel{0.81}}=\frac{100}{0.81}[/tex][tex]t^2=123.45679[/tex]Take the square root of both-side of the equation.
[tex]t\approx11.11[/tex]T= 11.11 seconds.
Hence, the period of the pendulum is 11.11 seconds.