Let \[ A = \begin{bmatrix} a & b & c \\ d & e & f \\ g & h & i \end{bmatrix}\]
the determinant can be computed by Laplace expansion along any row or column. Using the first row:
\[ \det(A) = a \cdot \det\begin{bmatrix}e&f\\h&i\end{bmatrix} - b \cdot \det\begin{bmatrix}d&f\\g&i\end{bmatrix} + c \cdot \det\begin{bmatrix}d&e\\g&h\end{bmatrix} \]which simplifies to:
\[ \det(A) = a(ei - fh) - b(di - fg) + c(dh - eg) \]row‑1 expansion: a(ei - fh) - b(di - fg) + c(dh - eg)