Let d: Deluxe tickets
Let r: Regular tickets
Set the system of equations:
6d + 1.5r = 822
d + r = 371
Isolate d:
d = 371 - r
Replace d in the first equation:
6 (371 - r) + 1.5r = 822
2226 - 6r + 1.5r = 822
-4.5r = 822 - 2226
-4.5r = -1404
r = -1404 / -4.5
r = 312
Replace r in the second equation:
d + 312 = 371
d = 371 - 312
d = 59
They sold 312 regular tickets and 59 deluxe tickets.