This is a problem with 2 variables, 2 unknowns.
Let x = Number of $28 tickets
Let y = Number of $40 tickets
Since we know there are 6000 seats in the theater
Since we know we want the revenue to be 187200
These are the 2 equations. You can solve by elimination, substitution or graphing. Either method would give you the solution:
There were 4400 $28 tickets sold and 1500 $40 tickets sold.