what is formula for density

Dimensional formula for relative density??

A total charge of 6.3 *10-8 C is placed on a 2.7 cm radius isolatedconducting sphere. The surface charge density is:
a) 7.6 *10-4 C/m2
b) 7.6 *10-4 C/m3
c) 6.9 *10-6 C/m2
d) 3.7 *10-7 C/m2
e) 2.5 *10-4 C/m3

Been searching for a formula for surface charge density in the bookbut i cant find it. i tried guessing on webassign but now im downto one more try. Please give me the formula and solve. Im gona needthe formula for later.

Estimate the average blood pressure in a person's foot, if thefootis 1.43 m below the aorta, where theaverage blood pressure is105 mm Hg.For the purposes of this estimate, assume the bloodisn'tflowing.
I'm not sure what formula to use.

Density of blood =1060