Waste of time #1

Discrete Mathematics Level pending

If $$n>2$$ and $$C_r$$ is the co-efficient of $$x^r$$ in $$(1+x)^n$$ then, $$(a-1)^2C_1-(a-2)^2C_2+....+(-1)^{n-1}(a-n)^2C_n=$$

×