Denote x2 by y.
(x2-3)7=(y-3)7
This is a binomial expansion in y, and you want the coefficient of y4 because y4=x8
You have 7 terms of (y-3) in (y-3)7. To get the fourth power of y, you need to choose y from four of the terms. The number of ways you can do this is the combinations of 7 things taken 4 at a time. This is:
7!/(4!3!)=35
So, the coefficient of x8 in the given expansion will be 210.
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