x = number of students who are members.
19-x is the number of people who are not members. x number of people paid $11 for tickets and (19-x) number of people paid $13.$11x+$13(19-x)=$209 for the total cost of the tickets.
$11x+ $247-$13x=$209 $11x-13x = $209 - $247
$-2x = $-38
divide -2 by both sides
x=19, meaning that all of the students are members.