Solution
= Solution
The <distance from a point to a plane> is the absolute residual of the plane equation divided by its normal's length. Here $\mathbf n\cdot\mathbf c=11$ and $|\mathbf n|=3$, so
$$
\boxed{d=\frac{|11-20|}{3}=3.}
$$
= Solution
The <distance from a point to a plane> is the absolute residual of the plane equation divided by its normal's length. Here $\mathbf n\cdot\mathbf c=11$ and $|\mathbf n|=3$, so
$$
\boxed{d=\frac{|11-20|}{3}=3.}
$$