The required pair of (x,y) values is found within the area bounded by the lines x + y =4, 7x + 3y = 21, the X- axis, and the Y-axis.
Procedure
Rewrite the first two lines as y = .......
You can then find the point of intersection of the first two lines by setting them equal to each other and solve for x and y.
This point is also within the required area.
Careful analysis reveals that the coordinates of this point are what is required to maximize f = 3x + 2y.
