Answer
$t=0,12,24$ for low tide and $t=6, 18$ for high tide
Work Step by Step
From the previous part (a), we have $d= -6.5 \cos (\dfrac{\pi}{6})t+10$
Here, Low Tide $d=3.5$
$3.5= -6.5 \cos (\dfrac{\pi}{6})t+10 \implies \cos (\dfrac{\pi}{6})t=1$
or, $2 \pi n=\dfrac{\pi}{6}t \implies 12n=t$
This gives: $t=0,12,24$
Here, High Tide $d=16.5$
$16.5= -6.5 \cos (\dfrac{\pi}{6})t+10 \implies \cos (\dfrac{\pi}{6})t=-1$
or, $\pi+2 \pi n=\dfrac{\pi}{6}t \implies 6+12n=t$
This gives: $t=6, 18$
