Aparking garage charges $3 for first two hours

If you park n cars for t hours, 
car 1: x = 3 + 2(t-1) 
others: 3/4 that amount, or 3/4 (n-1)*x 

So, the total amount will be 

f(n,t) = (3+2(t-1))(1 + (3/4)(n-1)) 
= (3n+1)(2t+1)/4 

So, 3 cars for t hours is 
f(3,t) = 5/2 (2t+1) 

So, let's check, say, f(3,4)=45/2=$22.50 

1st car: 3 + 2*3 = 9 
other 2 cars: each 3/4 of that amount, or 2*27/4 = $13.50 
Total: $22.50

Its 22.50 hope i helped