Coefficient of x - Help

This is a little computation heavy so I will outline the method.

First understand that you each term in the expansion is obtained by choosing one term from each of the brackets.

To form x^{\frac{n(n+1)}{2}-7} it means you are leaving out the variable terms in certain brackets and instead choosing the constants appearing in those brackets. So you have to add up the product of such constant terms. To illustrate:

You leave out x⁷ and so you get one of the terms as -7x^{\frac{n(n+1)}{2}-7}

Again, if you leave out x and x⁶, and get 6x^{\frac{n(n+1)}{2}-7}

Leaving out x² and x⁵, we get 10x^{\frac{n(n+1)}{2}-7}

Like this with x³ and x⁴

Further x,x² and x⁴...

In this way you add up and get the coeff

Thanks Sir, I got it. I was proceeding the same way, but didn't consider all those combinations.