In these types of problems, you will get two equations: the first equation relates to the dollars and the second equation relates to "how many".
OK, lets call advance tickets = a and door tickets = d. These are our unknowns.
The first equation is about the money. $7a+$9d=$5740.
The second equation is about how many. a+d=684.
We have 2 equations and 2 unknowns, so we can use substitution method to solve this thing! Let's rewrite the second equation a=684-d (you could also solve this by writing d=684-a, you choose).
Substituting our new second equation into our first equation, we get
$7(684-d)+$9d=$5740. Now using your skills to solve equations, here are the steps: