Chapter 6 - Section 6.2 - Factoring Trinomials of the Form x2+bx+c - Exercise Set - Page 429: 80

The last term factors must have the same sign because their product must be positive meaning that they must both be negative or both be positive. If one is negative and the other positive, the product will be negative, which will not fit the polynomial being factored.

In the form "x²+bx+c" if c is positive, then the last term factors must both be positive or negative.