Answer:
12x + 11y = 6569 (X is adult ticket, Y is senior ticket)
Y = 2x - 21
Now to plug it in:
12x + 11(2x-21) = 6569
12x+22x-231 = 6569
34x = 6569 + 231 = 6800
34x = 6800
x = 200
y = 379
200 adult tickets were sold and 379 senior tickets were sold.