Remember that a plane can be defined by just a point and its normal:

or where (a,b,c) is your normal and is a point.

We have the point (0, 0, 1) and we have the direction vector of the line (0, -1, 0). We have to find a vectorperpendicularto this (i.e. the normal). This is simple as all we have to do is find a vector such that: (remember, two vectors perpendicular to each other will give a dot product of 0).

So expand a bit: . Any solution will work. ex. (1, 0, 1). (In this case, b has to be equal to 0 since there's no other variable that can cancel it out).

So, plugging it all into , we get ...