Adult tickets for the school musical sold for $3.50 and student tickets sold for $2.50. three hundred twenty-one tickets for sold all together $937.50. how many of each kind of tickets were sold?

You cannot combine them in only one equation because they represent different things; so you will need two equations.
Now before we construct the two equations; lets set up our variables
Let a represent adult tickets
Let s represent student tickets
Now lets look at what we now about these two when thinking about them in money terms:
$3.50 per adult
$2.50 per student
Total made $937.50
Lets show it algebraically: 3.50a + 2.50s=$937.50
When thinking about the tickets in numbers we now the number sold were 321 so algebraically a + s = 321
Two equations; two variables; we can solve
a + s = 321
which is the same as
s= 321 - a
3.50a + 2.50s = 937.50
Since s = 321-a, we can substitute this into second equation:
3.50a + 2.50(321-a)=937.50
3.50a+802.5-2.50a=937.50
3.50a-2.50a+802.5=937.50
1.00a+802.5=937.50
a=135
Then s=321-135
s=186

This law is sometimes called the grouping property. as you can see, regardless of how the numbers are grouped, the answer is still 13. the associative law of multiplication is similar to the same law for addition. it says that no matter how you group numbers you are multiplying together, you will get the same answer.