Each of these equations represents a straight line. You find at least two values of x and y for each of these equations which satisfy the equation. Find integral values for x & y so that it is to easy to draw graph.
For example, For x + 2y = 4, when x = 2, y = 1 and when x = 4, y = 0.
These two sets (2,1) and (4,0) represent two points through which the line represented by x + 2y = 4.
Hence on graph draw a line through the points (2,1) and (4,0)
Similarly, points (2,1) and (6, -6) represent two points through which the line represented by 7x + 4y = 18 passes. Draw a line through these points.
You will see that the lines cut each other at only one point.
Hence this presents uniqueness of the solution.