The length of the string (l) is 150 cm and its linear mass density (μ) is 0.0035 kg/m. if the string oscillates with a frequency of 20 hz, determine the mass of the hanging mass

Assuming that the frequency is it's fundamental frequency we have that:
 n=1
If the string is being pulled by a hanging mass then we have that:

where M is the hanging object mass. Then we get that M is:

The answer to your is is c motion and shape