A fraction is positive in 2 conditions : a) Numerator and denominator both positive or b) Numerator and denominator both negative.
If one is positive and the other is negative then the fraction would be negative.
In your fraction numerator is a standard positive number, so it would be always positive. That means you are in condition (a) above so you need denominator to be positive.
Hope i said it well, because English isn't my mother language