
Answer:
 L = 17.94 m
Explanation:
The angular velocity of a simple pendulum is
     w = [tex]\sqrt{ \frac{g}{L} }[/tex]
the angular velocity is related to the period
     w = 2π / T
we substitute
     [tex]\frac{4\pi ^2 }{T^2} = \frac{g}{L}[/tex]
     L = [tex]\frac{g T^2}{4 \pi ^2}[/tex]
let's calculate
     L = 9.8 8.5² / 4π² Â
     L = 17.94 m