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

Let a represent adult tickets 
Let s represent student tickets 


3.50 a + 2.50 s = $937.50  eqation 1

three hundred twenty one tickets were sold altogether

a + s = 321   equation 2
s= 321 - a

we replace it in equation 1 so

3.50a + 2.50s = 937.50 
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 

and  s=321-135= =186 

Where are the functions?