Given the objective function l(x)=x1+x2 corresponding to l(x)=5,7,9 subject to the constraints:
x1-x2 less than or equal to 5;
-2x1+x2 less than or equal to 4;
x1 greater than or equal to 0 and x2 greater than or equal to 0.
The draw the contour lines (level curves)
