Obtain the radius of a sphere of zinc if its mass is 0.23 mg. The density of zinc is 7.14 g/mL.

The given formulas are:
d=m/v
v=(4/3)pi r^3

You have the formulas. Just substitute and solve them.

mass = volume x density. You have the mass (convert to grams) and density. Solve for volume.
Then volume = (4/3)*pi*r^3
Substitute for pi and volume, solve for r. Post your work if you get stuck.