For each of the following linear systems, obtain a solution by graphical methods, if possible. Explain the results from a geometrical standpoint.
a. x1 + 2x2 = 3,
x1 − x2 = 0.
b. x1 + 2x2 = 3,
2x1 + 4x2 = 6.
c. x1 + 2x2 = 0,
2x1 + 4x2 = 0.
d. 2x1 + x2 = −1,
4x1 + 2x2 = −2,
x1 − 3x2 = 5.