1.the two equations you have given are equations of planes, not lines.
2.if you have two coplanar lines that are not parallel to each other taking their cross product should give you a normal vector to the plane.
3. I assume that by M(x,y,z) you mean any point on the plane, you are allowed to take any point on the plane so taking one that lies on one of the lines that are on the plane is ok.
4. you are also allowed to take the point of intersection of the coplanar lines. It really makes no difference which point you choose as long as it is on the plane.