The easiest common denominator to find is the product of the denominators you have.
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The difference is ...
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<em>Additional comment</em>
The difference numerator and denominator have a common factor of 2, so can be reduced to 17/24. The denominator 24 could have been used as the common denominator from the start.
Using the <em>least common denominator</em> does not always result in a reduced sum or difference. If some other denominator is used (as here), then you can be assured that the result will need to be reduced to get to lowest terms.