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**undefined** 1f) 4!/(2!2!) is right

1g) it is 0 because 2+2 does not equal 6

2f) Note that y^4 is (y^2)^2 so this coefficient is nonzero since 1+2+1 = 4. You have it right, but it helps to write 2^2 instead of just 4, for clarity, as in 4!/(1!2!1!) * 2^2 = 48.

2g) 150 is right, again you can write more clearly as 6!/(1!4!1!) * (-1)^4 * 5^1 = 150.

I've helped you with the first two... you should be more confident about whether your third solution is right or wrong now..

Note that you can use CAS's to easily check your answers; Pari/GP is freeware (open source, in fact) and very good, Wolfram Alpha is online and also free.