The person's horizontal position is given by

and the time it takes for him to travel 56.6 m is

so your first computed time is the correct one.
The question requires a bit of careful reading, and I think there may be a mistake in the problem. The person's vertical velocity
at time
is

which tells us that he would reach the ground at about
. In this time, he would have traveled

But we're told that he is caught by a net at 56.6 m, which would mean that the net cannot have been placed at the same height from which he was launched. However, it's possible that the moment at which he was launched doesn't refer to the moment the cannon went off, but rather the moment at which the person left the muzzle of the cannon a fraction of a second after the cannon was set off. After this time, the person's initial vertical velocity
would have been a bit smaller than
.
Answer:
1.2 m/s
0.31 m
0.15 m
Explanation:
Time period is

Frequency is

Velocity is given by

The waves are traveling at 1.2 m/s
Amplitude is given by

Amplitude is 0.31 m
If d = 0.3 m

The amplitude would be 0.15 m. The speed would remain the same.