Multiply the binomials useing the foil method and simplify.
(b-3)(b+9)

a. b²-27
b. b²+12b-27
c. b²+6b-27
d. b²-6b-27

The foil method is the same as the method I've been showing you, except in a certain order (Firsts, Outsiders, Insiders, Lasts), so:
(b - 3)(b + 9)
Firsts: b × b = b^2
Outsiders: b × 9 = 9b
Insiders: -3 × b = -3b
Lasts: -3 × 9 = -27
So in total you get b^2 + 6b - 27, which is answer C. I hope this helps!