Describe the surface in double-struck r3 represented by the equation x + y = 9. this is the set {(x, 9 â’ x, z)|x is in double-struck r, z is in double-struck r} which is a vertical plane that intersects the xy-plane in the line y = 9 â’ x, z = 0. this is the set {(x, 9 â’ x, z)|x is in double-struck r, z is in double-struck r} which is a horizontal plane that intersects the xy-plane in the line y = 9 â’ x, z = 0. this is the set {(x, 9 â’ x, z)|x is in double-struck r, z is in double-struck r} which is a vertical plane that intersects the xz-plane in the line y = 9 â’ x, z = 0. this is the set {(x, 9 â’ x, z)|x is in double-struck r, z is in double-struck r} which is a horizontal plane that intersects the xz-plane in the line y = 9 â’ x, z = 0. this is the set {(x, y, 9 â’ x â’ y)|x is in double-struck r, y is in double-struck r} which is a vertical plane that intersects the xy-plane in the line y = 9 â’ x, z = 0.