If block d weighs 300 lb and block b weighs 275 lb determine the required weight of block c and the angle theta for equilibrium?

Set up a free body diagram. 

and by reason, Tcd = Tbd 

Tbd y = 275 - 300*sinθ 
Tcd y = Tc - 300*sin30 

Tbd x = 300*cosθ 
Tcdx = 300 * cos30 

Tbd^2 = (275 - 300*sinθ)^2 + (300*cosθ)^2 
Tcd^2 = (300*sin30)^2 + (300 * cos30)^2 

(275 - 300*sinθ)^2 + (300*cosθ)^2 = (300*sin30)^2 + (300 * cos30)^2 
etc.

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -