the temperature (in Celsius) $\displaystyle t$ hours after 9am was modeled by the function $\displaystyle T(t) = 20 + 6\sin \frac{\pi t}{12}$ find the average temperature from 9am to 9pm.

answer: about 24 C

what I did:$\displaystyle \frac{1}{21-9} \int_9^{21} + 6\sin \frac{\pi t}{12}dt$ which gives me about 17.299