Let the number of games they played last season be x.
The number of games they will play this season is 12 more than 1/3 of the number of games they played last season.
So we can write the equation:
21 = (1/3)x + 12
Subtract 12 from both sides:
9 = (1/3)x
Multiply both sides by 3:
27 = x
Therefore, they played 27 games last season.