Question: Computing coefficients of a differential polynomial

Compute the coefficients of
> a:=`@@`(D,6)(u)^3; 
#and
> b:=`@@`(D,5)(u)*`@@`(D,6)(u)*`@@`(D,7)(u);
#in the differential polynomial
> c:=`@@`(D,18)(u^3);.

 

Helppppppp :) TIA

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