To find the greatest common factor (GCF) of the expression \(42m - 28n\), we can first find the GCF of the numerical coefficients, which are 42 and 28.
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Finding the GCF of 42 and 28:
- The factors of 42 are: 1, 2, 3, 6, 7, 14, 21, 42
- The factors of 28 are: 1, 2, 4, 7, 14, 28
- The common factors are: 1, 2, 7, 14
- The greatest common factor is 14.
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Checking if the variables (m and n) have any common factors:
- The variables \(m\) and \(n\) are different, so they do not have any common factors.
Thus, the overall GCF of the expression \(42m - 28n\) is 14.
The correct answer is A: 14.