Answer:
when the coefficient of x^2 is negative
Step-by-step explanation:
Suppose if we have a trinomial x^2 - 5x+6
We can factor this trinomial by finding two factors whole product is +6 and add upto -5
Suppose we have a trinomial -x^2 +5x-6
Here factoring is not possible directly
so we need to factor out negative from x^2
To factor any trinomial x^2 should be positive
So we factor out -1 when the coefficient of x^2 is negative