* What is the frequency when the mass is hung? You mentioned a frequency of "Hz" but didn't specify the actual value.
* What is the mass of the "kg" that is hung? You mentioned a mass of "kg" but didn't specify the actual value.
* What is the elastic cord's spring constant (k)? You need this information to calculate the frequency.
Once you provide these details, I can help you calculate the frequency of the elastic cord when only the mass "kg" is hung from it.
