Subtract 2 seats each row you go back.
Row 1: 86
Row 2: 84
Row 3: 82
Row 4: 80
Row 5: 78
Row 6: 76
Row 7: 74
Row 8: 72
Row 9: 70
Row 10: 68
Row 11: 66
Row 12: 64
Row 13: 62
Row 14: 60
Row 15: 58
Row 16: 56
Row 17: 54
54 seats in Row 17.
Or, a simpler way:
Go back 16 rows, so multiply 2 by 16.
2 x 16 = 32
Subtract 32 from 86.
86-32 = 54
