For a standing wave on a string, the wavelength is equal to twice the length of the string:

In our problem, L=50.0 cm=0.50 m, therefore the wavelength of the wave is

And the speed of the wave is given by the product between the frequency and the wavelength of the wave:
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Answer:
8 electrons in the third energy level
Explanation:
From the description,the third energy level has 8 electron (represented by the small green balls you describe)
Here we will say that there is no external torque on the system so we will have

here we know that

where we know that

Also we know that

initial angular speed will be

now from above equation



now we have

so final speed will be 2.41 rad/s