You've failed because you failing becomes a statement rather than it becoming fact or what actually happened.
Answers:
a) 10 m
b) time=1.6 s, frquency=0.625 Hz
c) 6.25 m/s
Explanation:
a) If there is a crest at each dock and another three crests between the two docks, and the wavelength
is the distance between to crests; this means we have
in
:

Clearing
:


b) This part can be solved by a Rule of Three:
If 10 waves ---- 16 s
1 wave ----- 
Then:

This is the period of the wave
On the other hand, the frequency
of the wave has an inverse relation with its period
:


This is the frequency of the wave
c) The speed
of a wave is given by the following equation:


Finally:

Explanation:
By the second law of Newton we get the relation
F = ma
Explanation:
(a) Formula to calculate the density is as follows.

= 
= 
Now, calculate the charge as follows.

= 
=
C
or, = 101.06 nC
(b) For r = 6.50 cm, the value of charge will be calculated as follows.

= 
= 7.454 