12. Let vector $\overrightarrow{O A}=\{3,1\}, \overrightarrow{O B}=\{-1,2\}$, vector $\overrightarrow{O C} \perp \overrightarrow{O B}, \overrightarrow{B C} / / \overrightarrow{O A}$. Also, $\overrightarrow{O D}+\overrightarrow{O A}=\overrightarrow{O C}$, find $\overrightarrow{O D}$.
12. Let $\overrightarrow{O C}=\{x, y\}$, since $\overrightarrow{O C} \perp \overrightarrow{O B}$, we have $-x+2 y=0$. Also, $\overrightarrow{B C}=\{x+1, y-2\}, \overrightarrow{O A}=$ $\{3,1\}$, from $\overrightarrow{B C} / / \overrightarrow{O A}$ we get $x+1=3(y-2)$, which is $x-3 y+7=0$, so $x=14, y=7$. Therefore, $\overrightarrow{O D}=$ $\overrightarrow{O C}-\overrightarrow{O A}=\{1,6\}$.