Solve the System by the Addition Method $$\begin{array} { l } x + y = - 6 \\x - y = 12\end{array}$$

Solve the system by the addition method.
- $$\begin{array} { l } x + y = - 6 \\x - y = 12\end{array}$$

A) $\{ ( 6 , - 9 ) \}$
B) $\{ ( 6,3 ) \}$
C) $\{ ( 3,9 ) \}$
D) $\{ ( 3 , - 9 ) \}$