You want to factor each one of these functions and make it equal to 0. That’s what x intercept is, making y equal 0 and seeing where the function crosses the x axis.
x^2 - 9 = (x-3)(x+3) it has two zeros
x^2 - 6x + 9 = (x - 3)^2 it has one zero, 3
x^2 - 5x + 6 = (x - 3)(x - 2) it has two zeros
x^2 + x - 6 = (x + 3)(x - 2) it has two zeros
