This method only works for 3×3 matrices. Given a matrix A of order 3×3.
To apply sarrus rule, copy the first and second column of A to form fourth and fifth columns. The determinant of A is then obtained by adding the products of the three “DOWNWARD DIAGONALS” and subtracting the products of the three “UPWARD DIAGONALS” as shown


Thus, the determinant of 3 × 3 matrix A is given by the following
a₁b₂c₃ + b₁c₂a₃ + c₁a₂b₃ – a₃b₂c₁ – b₃c₂ a₁ – c₃ a₂ b₁

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Example on Sarrus Rule when finding determinant of 3x3 matrix
UTME 2014 Question 24
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