Answer: 27
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Explanation:
Check out the diagrams below. I posted two of them. Start with figure 1.
Notice how I've copied the first two columns and placed them on the right side of the matrix.
After that, I've highlighted the diagonals. The numbers in each diagonal grouping multiply together. I've used color coding to show this.
Once you find all those products, you add up the values. Figure 1 shows a result of 42. This isn't the final answer, but it helps us get there.
Figure 2 is a similar idea, but we're doing the off diagonals instead of the main diagonals. The result here is 15.
The last step is to subtract the results found in figures 1 and 2. We get 42-15 = 27 as the final answer.
You could use co-factor expansion as an alternative method. For me, I prefer this method of multiplying diagonals. Unfortunately the method only works for 3x3 matrices.
