The coefficient of x28in the expansion of 1+x3−x63nis
1
0
30C6
30C3
(1+x3−x630 =1+x31−x330 =30C0+30C1x31−x3+30C2x61−x32+…Obviously, each term will contain x3m,m∈N. But 28 is not divisible by 3. Therefore, there will be no term containing x28.