Find the equation of the plane which passes through the point (1,2,3) and contains the line of intersection of the planes 2x-y+z=4 and x+y+z=4.
You're guessing. You need to think geometrically here. You've got one point on the plane, and, given the equations you have, you could generate the equation for a line in the plane. But to write down the equation for a plane, you need a line normal, or perpendicular, to the plane. How can you get that line?