f(x) = -x^2 + 3x + 18
On the y-axis, x = 0. Thus, we have:
y = -0^2 + 3(0) + 18
y = 18
Thus, the y-intercept is (0, 18).
I am not sure what you mean by x-coordinates.
Suppose you mean the roots. On the x-axis, y = 0. We have:
0 = -x^2 + 3x + 18
0 = x^2 - 3x - 18
x^2 - 3x - 18 = 0
x^2 - 6x + 3x - 18 = 0
(x^2 - 6x) + (3x - 18) = 0
x(x - 6) + 3(x - 6) = 0
(x - 6)(x + 3) = 0
x - 6 = 0 or x + 3 = 0
x = 6 or x = -3