Matrices and Determinants

Let $A=\left(\begin{array}{rr}4 & -2 \\ \alpha & \beta\end{array}\right)$. If $\mathrm{A}^{2}+\gamma \mathrm{A}+18 \mathrm{I}=\mathrm{O}$, then $\operatorname{det}(\mathrm{A})$ is equal to _____________.