Typically, we look to multiply by the bottom number (the denominator) in order to get fractions to cancel out. If we multiply 1/6 by 6, we get just 1, which is why we do it.
In this problem, you have two different denominators, 2 and 4. If you multiply them all by 2, the term -3/4m will still be a fraction.
-3/4m * 2 = -3/2m
As a result, we must try 4 instead. This causes all of them to cease being a fraction. Even the one with 2 as a denominator.
