Find an integer value of b that makes x^2 + bx - 81 factorable

x² + bx - 81

there are 2 formula that fit this equation:

>> x² - a² = (x+a) (x-a)

or

>> x² + 2ax² + a² = (x+a) (x+a)

because the last value of our equation is negative value therefore we have to use the first formula >> x² - a² = (x+a) (x-a)

step:

x² + bx - 81

x² + bx - 9²

because we have to follow the first formula x² - a² therefore bx = 0

x² + bx - 9²

bx = 0

b = 0