First, let's define your variables!
x=cost of tickets for adults
y=cost of tickets for kids...
You know that they sold 900 tickets, which means the combination of x and y equals 900.
Therefore one equation is...
x+y=900
We also have to take care of the second equation...
We know that 13x represents the total cost of ADULT tickets
We know 7y represents the total cost for the children...
Which means...
13x+7y=11000
There are you equations! (bolded)
