Find the determinant

|2 -1 2|

|-4 0 1|

|-1 -1 -1|

(the vertical line at the front and back of each one are all suppose to be connected vertically)

$$\small{\text{ $ \left| \begin{array}{rrr} 2 &-1 &2 \\ -4 &0 &1 \\ -1 &-1 &-1 \\ \end{array} \right| $ }}\\\\\\ \begin{array}{ll} = & 2\cdot 0 \cdot (-1) + (-1)\cdot(-1)\cdot 1 + (-4)\cdot(-1)\cdot 2\\ &- (-1)\cdot 0 \cdot 2 - 2\cdot(-1)\cdot 1 - (-4)\cdot(-1)\cdot(-1)\\ = & 0 + 1 + 8\\ &-0 + 2 +4\\ = & 9\\ &+6\\ = 15 \end{array} $ }}$$

see also: https://www.khanacademy.org/math/precalculus/precalc-matrices/inverting_matrices/v/finding-the-determinant-of-a-3x3-matrix-method-1