Question 1170628
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Regarding part (a) in your post - - -  THERE  IS  NO  term/terms independent on x 
in the expression  (binomial expansion)   (x-3x)^6.



Also, there is no any connection between this expression  (x-3x)^6  and 
expressions of the part (b).



Be more accurate next time.


And in all times after the next.