The problem is asking you to express $S_n$ in sigma notation and to identify what symbols or expressions should be used for $x$ and $y$.

From the image, the general form of the sigma notation is given as:

$S_n = \sum_{x}^{y} T_k$

In sigma notation, $x$ typically represents the lower index (the starting point of the summation), and $y$ represents the upper index (the ending point). The expression inside the summation, $T_k$, represents the general term of the sum, where $k$ is the index variable.

To determine the appropriate values for $x$ and $y$, you would need additional information that clarifies the range over which the summation occurs.

Would you like help with interpreting any specific details of the problem, or is there more context available?