Answer
True
Work Step by Step
Step 1
Let denote the normal vectors of the given planes by $\vec{n}_1$ and $\vec{n}_2$.
Since the line $\ell$ belongs to both planes, we obtain that $\ell$ is orthogonal to both vectors $\vec{n}_1$ and $\vec{n}_2$, i.e. it is orthogonal to the plane determined by those two vectors.
What about the cross product $\vec{n}_1 \times \vec{n}_2$?
Step 2
We know that the cross product $\vec{n}_1 \times \vec{n}_2$ is the normal vector for the plane determined by vectors $\vec{n}_1$ and $\vec{n}_2$, so we get the line $\ell$ is actually parallel to the cross product $\vec{n}_1 \times \vec{n}_2$ because they are orthogonal to same plane.
