Factor: z^2 - 4z - 5.

1. The last term -5 is negative, so the two factors must have opposite signs, same as the related trinomial z squared + 4z - 5. 2. This time the factors must add to -4 instead of 4, so test 1 and -5 first: they sum to -4. 3. The pair 1 and -5 gives the correct sum of -4. 4. Use 1 and -5 as the last terms to get (z + 1)(z - 5), which is different from the factors of z squared + 4z - 5 even though the numbers 1 and 5 are reused. _Source: OpenStax Elementary Algebra (CC BY 4.0), Ch 7 "Factoring", section 7.2 Factor Quadratic Trinomials with Leading Coefficient 1_