a_n =n³  - 6n² + 11n - 6

Now put n = 1, 2, 3

     a₁ = 1³ - 6*1² + 11*1 - 6

=> a₁ = 1 - 6 + 11 - 6

=> a₁ = 12 - 12

=> a₁ = 0

a₂ = 2³ - 6*2² + 11*2 - 6

=> a₂ = 8 - 6*4 + 22 - 6

=> a₂ = 8 - 24 + 22 - 6

=> a₂ = 30 - 30

=> a₂ = 0

a₃ = 3³ - 6*3² + 11*3 - 6

=> a₃ = 27 - 6*9 + 33 - 6

=> a₃ = 27 - 54 + 33 - 6

=> a₃ = 60 - 50

=> a₃ = 0

Now put n = 4, 5

a₄ = 4³ - 6*4² + 11*4 - 6

=> a₄ = 64 - 6*16 + 44 - 6

=> a₄ = 64 - 96 + 44 - 6

=> a₄ = 108 - 102

=> a₄ = 6

=> a₄ > 0

Again 

a₅ = 5³ - 6*5² + 11*5 - 6

=> a₅ = 125 - 6*25 + 55 - 6

=> a₅ = 125 - 150 + 55 - 6

=> a₅ = 180 - 156

=> a₅ = 24

=> a₅ > 0

and so on.

Hense, the first three terms of this sequence are zero and all other terms are positive.