r + g = 36000.....r = 36000 - g
12r + 6g = 258,000
12(36000 - g) + 6g = 258000
432000 - 12g + 6g = 258000
-12g + 6g = 258000 - 432000
-6g = -174000
g = 174000/6
g = 29,000 <=== general admission seats sold
r = 36000 - g
r = 36000 - 29000
r = 7,000 <== reserved seats sold