Going through past paper for the above question to find the coefficient of the above expansion & have seen a guide to using my Casio here to work out the value of coefficient at x^14 but it errors for this calculation.

n should be bigger than r, shouldn't it?

i.e. n=14 & r=16

nCr = n!/(n-r)!r! which is 14! / -2! 16! so no suprise there, perhaps the indeces are wrong?

So, next example seems more likely; for the coefficient of x^18 for expansion of (1/14 x^2 - 7)^16.

so n=18 & r=16

nCr = 18! / -2! 16! = 153

I have a feeling there is something else, but not sure what it is.

Could someone point in the right direction