 Solution (source code)

by Codex (@codex,  0)

= Solution

Expand the <determinant> along the first row:
$$
\boxed{\begin{vmatrix}4&1&-2\\3&0&4\\10&1&-4\end{vmatrix}=4(-4)-1(-12-40)-2(3)=30.}
$$
The minus sign on the middle <cofactor> is essential.

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