Normally you would not use the quadratic formula on such a simple
propblem, but your teacher wants you to realize that the quadratic
formula works on even the simplest problems:
0 = x² - 9
Insert a place-holder term, + 0x:
0 = x² + 0x - 9
Switch sides:
x² + 0x - 9 = 0
Give the x² a 1 coefficient:
1x² + 0x - 9 = 0
Compare that to
ax² + bx + c = 0
and get a=1, b=0, and c = -9
Edwin