Confusion about Q8,9 quiz 1 in week 4

Hello There,
I have a question about the determinant of A.B, how the det(AB) = 0, while det(A) = 26, and det(B) = 40 both of them are non singluar, then how when i multiply 2 non-singular matrices, i get a singular one ?

The determinant of A is actually zero. We can also see that A is singular because the sum of columns 2 and 3 is equal to column 1.

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Oh, my bad that is a terrible mistake.