What's wrong with this approach?

I tried solving this question considering the pentagon inside as a regular pentagon, but the answer didn’t match any of the options. I get 108 as value of “a”. Greg solved this by considering the whole 9 sided polygon as a regular polygon, but initially it didn’t make much sense to me considering the whole polygon as regular.

Can you please share the link for this class

Sure
https://www.gregmat.com/class/practice-all-geometry-concepts