The first step is to find the cofactors of each of the elements in the first column. We then multiply by the elements of the first row and assign plus and minus in the order:

plus, minus, plus, minus

`| (7,4,2,0), (6,3,-1,2), (4,6,2,5), (8,2,-7,1) |`

`7 | (3,-1,2), (6,2,5), (2,-7,1) | ` `-6| (4,2,0), (6,2,5), (2,-7,1) | ` `+4 | (4,2,0), (3,-1,2), (2,-7,1) | ` `-8 | (4,2,0), (3,-1,2), (6,2,5) |`

Now, we expand out each of those 3 × 3 cofactors using the method that we saw before:

`| (3,-1,2), (6,2,5), (2,-7,1) |=15`

`| (4,2,0), (6,2,5), (2,-7,1) |=156`

`| (4,2,0), (3,-1,2), (2,-7,1) |=54`

` | (4,2,0), (3,-1,2), (6,2,5) |=-42`

Now, putting it all together, we have:

`| (7,4,2,0), (6,3,-1,2), (4,6,2,5), (8,2,-7,1) |`

`=7xx15 - 6xx156` ` + 4xx54` ` -8xx-42`

`=-279`

Easy to understand math videos:
MathTutorDVD.com