Using the same principles as multiplying a negative whole number by a positive whole number, where the result would be a negative, the product of a (positive) fraction and a negative whole number...
try to memorise thispositiveXpositive=positiveNegativeXpositive=negativepositiveXnegative=negativenegativeXnegative=positivea - c -ac ---- 0 ----- = --------b d bd
In the same way that you would answer a fraction subtract another fraction. The result could be either positive or negative, just as is the case with negative whole numbers.