We know that a twovariable first degree equation has infinite solutions. Each of these solutions can be represented by an ordered pair (x, y).
Having two ordered pairs of an equation, we can graph them on a Cartesian plane, determining, through the line that joins them, the set of solutions of this equation. Example:
 Graph the equation x + y = 4.
Initially, we choose two ordered pairs that solve this equation.
1st pair: THE (4, 0)
2nd pair: B (0, 4)
We then represent these points on a Cartesian plane.

Finally, we join the points THE and B, determining the line r, which contains all points solutions of the equation.
The straight r it's called straight support of the equation graph.
Next: Equation Systems