Solution
= Solution
A <normal vector> to the <plane> spanned by $\mathbf a,\mathbf b$ is their <cross product>:
$$
\mathbf a\times\mathbf b=(-5,0,-5),\qquad |\mathbf a\times\mathbf b|=5\sqrt2.
$$
Dividing by this <Euclidean norm> gives the two possible choices of <unit normal>,
$$
\boxed{\mathbf n=\pm\frac{(1,0,1)}{\sqrt2}.}
$$