D(t) = -4t^3 + 40t^2 + 500t = 4088
4t^3 - 40t^2 - 500t + 4088 = 0
t = 7
Therefore, the plane has flown 4088 km 7 hours after it took off.
<u>Answer</u>
<em>The total displacement of the object is 320 m</em>
<u>Solution-</u>
From the formulas of mechanics,

Where,
s = displacement
u = initial velocity = 0 ( ∵ As the body was in rest in the beginning )
t = time taken = 8 s
a = acceleration = -10 m/s² ( ∵ -ve is because of the downward motion)
Putting all the values,
( ∵ -ve displacement means it is in downwards)