Use the simple method to solve the following problem:
max z = 2x1 - x2 + x3
subject to: 3x1 + x2 + x3 ≤ 60
x1 - x2 + 2x3 ≤ 10
x1 + x2 - x3 ≤ 20
x1 . x2 , x3 ≥ 0