Answer
$B(t) = 4.0+0.35~sin[\frac{2\pi}{5.4}~t]$
Work Step by Step
The average brightness is 4.0.
The amplitude of the graph is 0.35.
The period is 5.4 days.
We can use these values to write a function that models the brightness:
$B(t) = 4.0+0.35~sin[\frac{2\pi}{5.4}~t]$
