That's a real pity. I don't know. I asked my classmates. She said that the answer thus obtained broadened the scope of the answer. ...

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.