(a)
The point of intersection of $\overrightarrow{\mathbf{r}} \times \overrightarrow{\mathbf{a}} = \overrightarrow{\mathbf{b}} \times \overrightarrow{\mathbf{a}}$ and $\overrightarrow{\mathbf{r}} \times \overrightarrow{\mathbf{b}} = \overrightarrow{\mathbf{a}} \times \overrightarrow{\mathbf{b}}$ is $\overrightarrow{\mathbf{r}} = \overrightarrow{\mathbf{a}} + \overrightarrow{\mathbf{b}}$
$\therefore\ \overrightarrow{\mathbf{r}} = \left( \widehat{\dot{\mathbf{i}}} + \widehat{\dot{\mathbf{j}}} \right) + (2\widehat{\dot{\mathbf{i}}} - \widehat{\mathbf{k}}) = 3\widehat{\dot{\mathbf{i}}} + \widehat{\dot{\mathbf{j}}} - \widehat{\mathbf{k}}$