Answer:
b = 10
Explanation:
expand expression and compare like terms.
(r + a)(r - 15) ← expand using FOIL
= r² - 15r + ar - 15a ← factor out r from second/ third terms
= r² + r(a - 15) - 15a
compare with r² - br - 75
then
- 15a = - 75 ( divide both sides by - 15 )
a = 5
and
a - 15 = - b ← substitute a = 5
5 - 15 = - b
- 10 = - b ( multiply both sides by - 1 )
10 = b