PLEASE COMPLETE THE QUESTION ....
Answer:
21 child tickets.
Step-by-step explanation:
Let x represent number of adult tickets and y represent number of child tickets.
We have been given that on Thursday, three times as many adult tickets as child tickets were sold. We can represent this information in an equation as:

We have been given that at a the movie theater, child admission is $6.10 and adult admission is $9.20.The all tickets were sold for a total of $707.70. We can represent this information in an equation as:
Upon substituting equation (1) in equation (2), we will get:




Therefore, 21 child tickets were sold on Thursday.
"What do I want to find out?" is the first step.
Factoring out x
x(-3x^2+15x+42) now factor parenthetical expression
x(-3x^2+21x-6x+42)
x(-3x(x-7)-6(x-7))
x(-3x-6)(x-7) we can factor out -3 from first parentheses
-3x(x+2)(x-7)