x+2≥0x≥-2
x-y≤1x≤y+1
-2≤x≤y+1
y+1≥-2
y≥-3
x+2y≤4
-2+2y≤x+2y≤y+1+2y≤4
x取最大值y+1,x+2y≤4成立
3y+1≤4
y≤1
-3≤y≤1
z=3x-y
-2-y≤z≤3(y+1)-y=2y+3
z≤2+3=5
z的最大值为5.