You write "the directrix of a line...". A directrix is the line perpendicular to the axis of symmetry of a parabola. So, unless the line that passes through point A is actually a parabola, you are just looking for a line that is perpendicular to another line. Next, which line is supposed to be parallel to the x-axis? If A is a point, you cannot find a vector that is perpendicular to it. You need to find a vector that is perpendicular to some line. So, you can write $\displaystyle B(x,y,z)$ is another point on the line that passes through $\displaystyle A$. Then, since this line is parallel to the $\displaystyle x$-axis, you know the difference of position vectors $\displaystyle \vec{B}-\vec{A}$ is some scalar multiple of $\displaystyle (1,0,0)$. Hence, $\displaystyle y=-2,z=3$. This gives a vector in the direction of your line that passes through point A to be $\displaystyle (1,0,0)$. So, a line perpendicular to that could be $\displaystyle (0,1,0)$, it could be $\displaystyle (0,0,1)$. Since you are not given a parabola, it is difficult to know the specific perpendicular line the question is looking for. There is no axis of symmetry to compare it to.