A star is estimated to have a mass of 2.0 ✕ 1036 kg. Assuming it to be a sphere of average radius 6.8 ✕ 105 km, calculate the average density of the star in units of grams per cubic centimeter.
mass = 2.0 x 10^36 kg = 2.0 x 10^39 grams.
radius = 6.8 x 10^5 km x (1,000 m/km) x (100 cm/m) = 6.8 x 10^10 cm
volume of a sphere is 4/3*pi*r^3 . Plug in the r from above in cm and solve for volume in cc.
Then density in g/cc = mass in g/volume in cc
Post your work if you get stuck.