Apollo had to pay 3 * 6 = <<3*6=18>>18 golden apples for the first six months.
For the second six months, he had to pay twice that amount, or 18 * 2 = <<18*2=36>>36 golden apples.
Thus, Apollo had to pay 18 + 36 = <<18+36=54>>54 golden apples for a year of chariot wheels.
#### 54