When point $\mathrm { P } ( 2 , - 1 )$ is mapped to point Q by the linear transformation represented by the matrix $\left( \begin{array} { r r } 1 & 2 \\ - 2 & 1 \end{array} \right)$, what is the slope of line PQ? [3 points]\\
(1) 1\\
(2) 2\\
(3) 3\\
(4) 4\\
(5) 5