If F of X equal three X -, a / X - 4 and F of 6 = 0, what's the value of a? So if we have F of 6 equaling 0? Remember this is really really a point. This is saying when X is 6, our Y is 0 or our F of X equals 0. So when X is 6, so we're going to put zero in for the F of X and we're going to put in six for the X and we're going to solve for the a. So F of X equal our output or equivalent of our Y SO60X was 6. Our output was the zero 6 - 4. Oops, 3 * 618 6 - 4 two. When we cross multiply, 0 * 2 is 0, so 18 -. A so a is 18. Now the second part says where is F not defined because it's a fraction. We know that the denominator can never ever equal 0, so X can never equal 4.