$b)$ Say, it can be a repeated characters string or non repeated character string or combination of repeated and non repeated character string
Firstly when always vowels are repeating
Case $1$: Say all vowel are repeating and all non vowel are repeating ${\color{Blue}{----}}{\color{Orange} {--}}$=$\binom{5}{1}\binom{21}{1}\frac{6!}{4!2!}=1575$
Case $2$: Say, all vowel are repeating and 1 consonant is repeating 3 times${\color{Blue}{---}}{\color{Cyan} -}{\color{Orange} {--}}$$\binom{5}{1}\binom{21}{1}\binom{20}{1}\frac{6!}{3!2!}$
$=5\times 21\times 20\times \frac{6\times 5\times 4}{2}=126000$
Case $3$: Say, 2 consonant are repeating 2 times=${\color{Blue} {--}}{\color{Teal} {--}}{\color{Orange} {--}}$$\binom{5}{1}\times \binom{21}{2}\frac{6!}{2!\times 2!\times 2!}=94500$
Case $4$: Say, no consonant are repeating$\binom{5}{1}\times \binom{21}{4}\frac{6!}{2!}=10,773,000$
Case $5$: Say, if there are three consonant , where $2$ repeating and $2$ non repeating ${\color{Blue} {--}}{\color{Magenta} {-}}{\color{DarkGreen} {-}}{\color{Orange} {--}}$
$\binom{5}{1}\times \binom{21}{1}\times \binom{20}{1}\times \binom{19}{1}\times \frac{6!}{2!\times 2!}=7,182,000$
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All these 5 cases when no vowels are repeating
Case $1$: Say all vowel are non repeating and all non vowel are repeating$\binom{5}{2}\binom{21}{1}\frac{6!}{4!}=6300$
Case $2$: Say, no vowel are repeating and 1 consonant is repeating 3 times=$\binom{5}{2}\binom{21}{1}\binom{20}{1}\frac{6!}{3!}=504000$
Case $3$: Say, $2$ consonant are repeating $2$ times=$\binom{5}{2}\binom{21}{2}\frac{6!}{2!2!}=378000$
Case $4$: Say, no consonant are repeating=$\binom{5}{2}\binom{21}{4}\times 6!=43,092,000$
Case $5$: Say, if there are three consonant , where $2$ repeating and $2$ non repeating=$\binom{5}{2}\times \binom{21}{1}\binom{20}{1}\binom{19}{1}\frac{6!}{2!}=28,728,000$
Now , summation of all these will be 90,883,800