Step 1 is to locate the point (1,2). You start at the origin, then move 1 unit to the right and 2 units up. Alternatively, you can draw a vertical line through 1 on the x axis and a horizontal line through 2 on the y axis. The horizontal and vertical line cross at (1,2)
Step 2 is to then determine which lines go through (1,2). The two lines in question are line A and line C. So the intersection of line A and line C is the point (1,2). This point satisfies the equations that make up these lines. So that is why it is considered a solution to just these two equations only (ignore the other lines).
