1. If 31 waves pass by a point in 1.7 s, what's the frequency?

my answer: 18.2 Hz

2. if a crest with an amplitude of 7cm interferes with a trough with an amplitude of 10 cm, the resulting wave has an amplitude of:

my answer: -3

yes

yes

To find the frequency in question 1, you need to use the formula:

frequency = number of waves / time

In this case, you are given that 31 waves pass by a point in 1.7 seconds. Plugging this into the formula, you get:

frequency = 31 waves / 1.7 s = 18.24 Hz (rounded to two decimal places)

So, the correct answer is approximately 18.24 Hz. It seems like you rounded it down to 18.2 Hz, which is quite close.

In question 2, when a crest with an amplitude of 7 cm interferes with a trough with an amplitude of 10 cm, the resulting wave will have an amplitude equal to the difference between the two amplitudes. Mathematically, you subtract the amplitude of the trough from the amplitude of the crest:

resulting amplitude = amplitude of crest - amplitude of trough
resulting amplitude = 7 cm - 10 cm = -3 cm

So, the correct answer is -3 cm. It looks like you correctly calculated the resulting amplitude in this case.