GATE2015-61

Question

A system is represented in state-space as $\dot{X}=AX+BU,$ where $A=\begin{bmatrix} 1 & 2 \\ \alpha & 6\end{bmatrix}$ and $B=\begin{bmatrix} 1 \\ 1\end{bmatrix}$. The value of $\alpha$ for which the system is not controllable is _____________.