There is this question asking for a coeffecient of a certain expansion, however I dont seem to be getting the correct answer.

With the expansion of

find the coefficient of

so what I have done is

Meaning

thus

Thus the coefficient of should be

and then it should be multiplied by

thus <- which I think should be the binomial coefficient of

However, i learned that the answer is -12

could anyone tell me where I have went wrong?