Đáp án:
\( \Delta \parallel \vec{a} = (1, -2, 3), \quad mpP \perp \vec{n}_\pi = (3, 1, 2). \)
\( \sin \varphi = \left| \cos (\vec{a}, \vec{n}_P) \right| = \frac{\left| 3 -2 +6 \right|}{\sqrt{14} \cdot \sqrt{14}} = \frac{1}{2} \Rightarrow \varphi = 30^\circ. \)