well in the power's formula N which constant goes to infinity how could this be ? i see in the question that N=7 which is constant,then should N or KN go to infinity ?
For periodic signals you can use
where $N$ is the period of the signal. I'm sure you can take it from here.
Also take a look at this question and its answer where the equivalence of $(1)$ and the formula in your question is discussed.