Find perp vec

Let \vec{u} = \langle t,2t,3 \rangle and let \vec{v} = \langle 3,2,1 \rangle.
Find a value of t such that \vec{u} is perpendicular to \vec{v} or explain why no such \vec{t} exists.

It doesn’t exist. The dot product between the two vectors is not 0.

Well, it might be zero for some values of t but not for others.

The values are perpendicular when:

I can’t disagree with this. Still, one must wonder where the result came from?