Let's work out the first few products:
n=1: x-1 [n(n+1)/2=1]
n=2: x2-3x+2 [n(n+1)/2=3]
n=3: x3-6x2+8x-6 [n(n+1)/2=6]
n=4: x4-10x3+32x2-38x+24 [n(n+1)/2=10]
So there's pattern: the coefficient of xn-1 is -n(n+1)/2 (the negative sum of natural numbers up to n).
When n=10, therefore, the coefficient of x9 is -10×11/2=-55.