Answer
$aei+bfg+cdh-ceg-afh-bdi$
Work Step by Step
Consider the $3\times 3$ matrix:
$A=\begin{bmatrix}a&b&c\\d&e&f\\g&h&i\end{bmatrix}$
Compute its determinant:
$det A=\begin{vmatrix}a&b&c\\d&e&f\\g&h&i\end{vmatrix}$
$=aei+bfg+cdh-ceg-afh-bdi$
