How do you factor b^2-81

The middle number is 0 and the last is - 81Factoring means we want something like (b+_) (b+_) We need two numbers that add together to get and Multiply to get - 81which are 9 and - 9: 9+-9 = 0 and 9*-9 = - 81

Fill in the blanks in

(b+_) (b+_)

with 9 and - 9 to get ... (b+9) (b-9)

The answer is : (b+9) (b-9)