Answer:
The bracket is the binomial.
The definition is: [tex]\binom{n}{k} = \frac{n!}{k!\cdot (n-k)!}[/tex]
Note that [tex]\binom{n}{k} = \binom{n}{n-k}[/tex]
The meaning is "if we have n elements, how many ways are there to choose k of them?" From this definition it should be obvious that [tex]\binom{n}{k} = \binom{n}{n-k}[/tex] - if you're picking k, it's just as many ways as choosing the (n-k) that you're not picking.
The coefficient can be found by realizing that to get x^9, we need to choose (from the full multiplication):
- 9 brackets to provide x
- 2 brackets to provide the number 3
There's [tex]\binom{11}{9}[/tex] ways to do that, so the component for x^9 will be:
[tex]\binom{11}{9} \cdot 3^2 \cdot x^9 = \frac{11!}{2!\cdot 9!} \cdot 3^2 \cdot x^9 = \frac{10\cdot 11}{2} \cdot 3^2 \cdot x^9 = 55 \cdot 9 \cdot x^9 = 495x^9[/tex]
So:
a = 2
Coefficient is 495
