What do the following equations represent in r3? match the two sets of letters: a. a vertical plane b. a horizontal plane c. a plane which is neither vertical nor horizontal b \displaystyle{b}b a. −9x+6y=4 b. x=6 c. y=−7 a \displaystyle{a}a d. z=0 a "horizontal" plane is parallel to the xy plane. it's normal vector is parallel to the normal vector of the xy plane. a "perpendicular" plane has a normal vector perpendicular to the normal vector for the xy plane hence its normal vector lies in the xy plane