$\"\"$

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The first equation is c(AB)

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$\"\"$

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$\"\"$

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$\"\"$

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Multiply the matrix AB elements by 3

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$\"\"$

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$\"\"$ $\"\"$

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The second equation is $\"\"$ .

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$\"\"$

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Multiply the matrix B elements by 3

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$\"\"$

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$\"\"$ (Substitute matrix A)

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Multiply columns by rows.

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$\"\"$

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$\"\"$ $\"\"$

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The solution of equation c(AB) =A(cB) is True.