I have a geometry problem I'm working out.

I have a heptagon with a circumcircular radius of 400 units. I am aware that I can calculate most other features of the heptagon using this information here https://rechneronline.de/pi/heptagon.php , but that's not particularly what I'm most interested in. Essentially my dilemma is pic related, I want to find a and b if c is already known, and I want to do this for each of the 7 points, except for the top one (which is already 400 up, 0 across). What formulas would be most useful for this? I don't think that the Pythagorean Theorem can help me until I get more data, since I only know c at the moment.