Factor: r^2 - 8rs - 9s^2.

1. Since the last term has s squared, the second term of each binomial factor must contain s. 2. Find two numbers that multiply to -9 and add to -8. 3. Testing the factor pairs of -9, the pair 1 and -9 multiplies to -9 and adds to -8. 4. Use 1 and -9 as the coefficients of s to get (r + s)(r - 9s), and check: r squared - 9rs + rs - 9s squared gives back r squared - 8rs - 9s squared. _Source: OpenStax Elementary Algebra (CC BY 4.0), Ch 7 "Factoring", section 7.2 Factor Quadratic Trinomials with Leading Coefficient 1_