Parc c of Exercise 4.3.12 in Shifrin and Adams' Linear Algebra: a Geometric Approach says
Let $V \subset \mathbb{R}^3$ be the subspace defined by $$V = \{(x_1, x_2, x_3): x_1 - x_2 + x_3 =0\}.$$
Find the standard matrix for rotation of $V$ through angle $\pi/6$ (as viewed from high above).
What does it mean to rotate $V$ through angle $\pi/6$ (as viewed from high above)?
V is a plane, in $\mathbb{R}^{3}$, so viewing this from far enough to see a rotation of an infinite thing is kind of confusing, so maybe pretend you mark a square on it with different colored sides and step back and see how the square moves after a rotation by $\pi/6$ counterclockwise.