Explanation:
let x be the number of adults and y the number of childrens
x+y=548 people because thats the whole number of sold tickets
and
6.50*x+3.50*y=2881 dollars in total
let separate x from the first formula
x=548-y
and replace it at the second equation
6.50*(548-y)+3.50y=2881
6.50*548-6.50y+3.50y=2881
3562-3y=2881
3y=681
y=227
so x=548-y=548-227=321
there were sold 321 adult tickets and 227 child's tickets.