Vector Algebra

The set of all \(\alpha\), for which the vectors \(\vec{a}=\alpha t \hat{i}+6 \hat{j}-3 \hat{k}\) and \(\vec{b}=t \hat{i}-2 \hat{j}-2 \alpha t \hat{k}\) are inclined at an obtuse angle for all \(t \in \mathbb{R}\), is