how would you find the position of the 2nd resonance in a one closed sided pipe
This is a confusing question you're asking. Resonance is when a periotic force is applied to a body which is the same or very near the natural frequency of the body that body will vibrate with a very large amplitude. I don't think its possible to have two positions of resonance but if there is I don't think you need to know about it for the leaving cert. Hope this helps
do you mean the second harmonic/overtone?
if by 2nd resonance you mean harmonic/overtone. Then these are multiples of the certain frequency. For example if you have a frequency f the 1st harmonic will be 2f and so on