Answer:
990 cars
Step-by-step explanation:
Find how many cars pass through at 6am, t=0:
r(0)= 300+600(0)-90(0²) = 300
Find how many cars pass through at 9am, t=3:
r(3)= 300+600(3)-900(3²)=300+1800-810=1290
Find the difference (how many cars pass between 6am and 9am): 1290-300=990