The graph shows two lines in the standard Cartesian coordinate plane, representing a system of two linear equations. The lines intersect at a single point.
[Graph description: A coordinate plane with two lines. Line 1 passes through (0,3) and (3,0). Line 2 passes through (0,7) and (2,1). The lines intersect at (2,1).]
If the two equations in the system are added together to form a third equation, which of the following statements must be true about the graph of this third equation?
It must pass through the origin (0,0).
It must be parallel to one of the original lines.
It must pass through the intersection point of the original two lines.
It must have a positive y-intercept.
