What should go to infinity in power's formula N or the repetition KN? I have this question :

I should now calculate power of y[n], where the formula

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
$$P_y=\frac{1}{N}\sum_{n=0}^{N-1}|y[n]|^2\tag{1}$$
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.