Given matrices \( A \) and \( B \) where:
\[
A = \begin{bmatrix}
1 & 2 \\
2 & 1
\end{bmatrix}, \quad
B = \begin{bmatrix}
x & y \\
1 & 2
\end{bmatrix}
\]
and the condition:
\[
(A + B)(A - B) = A^2 - B^2
\]
If matrix \( C \) is defined as:
\[
C = \begin{bmatrix}
x & 2 \\
1 & y
\end{bmatrix}
\]
then the trace of \( C \) is:
\[
\text{Tr}(C) = x + y
\]