1. I suppose each star is going to be affected by a centripetal force of Gm^2/d^2, where m is the mass of the stars and d the distance between them. Then it's a case of plugging this into the centripetal force formula F = mv^2/(d/2) and solving to find v. Once you have v you can find out the orbital period using the velocity formula v = pi*d/t and solving for t.

2. Not sure, never done questions on this. I suppose you use c = lambda*f to find what the velocity is and then subtract this from c. Not sure.

What symbols are confusing you? km is length, kg mass, nm is length, THz is frequency.