Can someone please explain to me how to solve a summation with subscripts.

This just means add up the individual terms (and divide the result by n).  For example 

$$If the $X$'s were $X_1,X_2,X_3$ then $\sum_{k=1}^3X_k = X_1+X_2+X_3$\\\\ Sometimes you might have a function to sum, rather than a set of data; and sometimes that summation might simplify. For example\\ $$ \sum_{k=1}^nk^2=\frac{n(n+1)(2n+1)}{6}$$

 I should perhaps expand that last one a little:

\sum_{k=1}^nk^2=1^2+2^2+3^2+...+n^2=\frac{n(n+1)(2n+1)}{6}$$

Could you provide an example of what you want to do??

Let's take the Law of Large Numbers for instance: