Geometric Average

Hi John Can you explain this to me in general. I have looked back at the valuation of security lectures and cannot seem to find the part covering this. My thought was add up the variables and then hit it with the Nth root of periods. I am having difficulty on on of the practise questions regarding dividend growth over 4 yr period. I just want to be sure I am doing it correctly first before I post the question here looking for help. Thank you.