What two numbers can be multiplied to be - 6 and added to be - 5

One, but only one of them will be negative: only a negative and a positive number will give a negative product.

let's try 6 and 1: one of them negative, one positive:

if we multiply - 6 and 1 we get - 6, and if we add them we get - 5: this is the correct answer!

-2x3 (vise-versa) = - 6

so ... - 6+1 = - 5