Density = mass/volume
Plug in your Provided Values
v=volume
18g/cm^3 = 4.5g/v. Multiple both sides by v
v(18g/cm^3) = 4.5g Divide both sides by 18g/cm^3.
v = 4.5g/(18g/cm^3) Your grams cancel out leaving 4.5/18 cm^3 as your volume which is also. .25 cm^3.
Hope this helps!