9. The Yangtze Farm has a pasture, where grass grows uniformly every day. If 24 cows are grazed on the pasture, they will finish the grass in 6 days; if only 21 cows are grazed, it will take 8 days to finish the grass. How many days will it take to finish the grass if 36 cows are grazed?
Assuming one cow eats 1 unit of grass per day, the new grass grows daily: $(21 \times 8 - 24 \times 6) \div (8 - 6) = 12$  
Original grass amount: $(21 - 12) \times 8 = 72$  
Therefore, $72 \div (36 - 12) = 3$ (days)