I have a question about polygons. Here's how it goes:

Attachment 19017

I know that the sum of interior angles of the regular nonagon is 180º(n-2)=

180n -360

= 180(9) -360

=1260º

So each of the interior angle is 1260º/9 = 140º

angle PCD + angle BCD = 180º (adjacent angles on a straight line)

angle PCD = 180º - 140º

angle PCD = 40º

So that explains part a).

Can you please give me tips on part b)?