Draw the feasible region, and find that three poles are (2,0) (0,1) (1/2,3) and bring them into the target equation z=3x-y, and the discovery range is-3/2,6.

Or three simultaneous equations (1)x+2y=2, 2x+y=4(2)x+2y=2, 4x-y=- 1(3)2x+y=4, 4x-y=- 1.

The final answer is a.