What is the density of a marble with mass 35g and volume 70mL?

Assuming it is a round marble (spherical actually) one first has to weigh it. Next one has to measure its diameter using a micrometer or a vernier scale. Halve the diameter to obtain the radius R. The volume can then be calculated by using the equation 4/3 x pi x R x R x R. Pi can be taken as being 22/7 or 3.142

Density is mass divided by volume and on the surface of the earth, mass and weight are usually regarded as being the same (not strictly true but near enough)