Correct answer: 2x+y= 1.5(x+y)-0.5(-x+y).
∫ 1≤x+y≤2
∴ 1.5≤ 1.5(x+y)≤3
∫- 1 ≤- x+y≤ 1
∴ -0.5≤-0.5(-x+y)≤0.5
∴ 1≤2x+y = 1.5(x+y)-0.5(-x+y)≤3.5
So 1≤f(2, 1)≤3.5.