(i)

Since the pendulum swings back and forth 10 times in 5π seconds, the pendulum will take π/2 seconds to swing back and forth once.

Period of swinging
= Time taken to swing back and forth once
= π/2

The variable b in d = 20 cos (bt) represents the frequency of the swinging which is given by

b
= Frequency
= 2π / Period
= 2π / (π/2)
= 4

(ii)

d = 20 cos (4t)

This is 4 cycles in a time span of 2π seconds.
We are only required to draw 2 cycles. The maximum height attained is 20 m, while the minimum height attained is -20 m.

You know how to draw the cosine graph correct? It starts from the highest point first before going down all the way and then going up all the way to complete one cycle. Then, we draw one such wave one more time.

(iii)

In the previous case, d = 20 when t = 0.

If d = 0 when t = 0, a sine graph might be more fitting.