Two sloping roofs of a hall lie in the planes 2x - y + 2z = 5 and x - 4y + 8z = 3, with coordinates…

The angle between planes has $\cos\theta = \dfrac{\lvert \vec{n}_1 \cdot \vec{n}_2 \rvert}{\lvert \vec{n}_1 \rvert \lvert \vec{n}_2 \rvert}$. The angle between a line and a plane has $\sin\varphi = \dfrac{\lvert \vec{b} \cdot \vec{n} \rvert}{\lvert \vec{b} \rvert \lvert \vec{n} \rvert}$. A line is parallel to a plane when $\vec{b} \cdot \vec{n} = 0$.