1- Leave the 1st case empty
2- Write on the 1st row each element of the trinomial in each case
3-Write on the 1st column in each case each of the element of the binomial
       ║     y²          -3y         9
â•‘
y       ║     y³          -3y²         9y
â•‘
3       ║    3y²          -9y          27
â•‘
  TOTAL║(y³+3y²) +(-3y²-9y)  + ( 9y+27)
If you add the Total you will get: (A) Y³+27