Aneutron star has a density of about 5.9 10 17 kg/m3.

what would be the approximate mass of a 1-centimeter cube (a cube 1 cm on all sides)?

The answer is, 5.9 10 ^11 kg (1.3 trillion pounds)

Approximate mass = 5.9*10^11 kg

Density = Mass/Volume
5.9*10^17 kg/m^3 = (unknown=Mass)/1 cm^3 (this must be converted first into m^3 to have unit consistency)
1 cm^3 (1/100)^3 = 1*10^-6 m^3

(5.9 * 10 ^17 kg/m^3)(1*10^-6 m^3) = Mass
5.9 * 10 ^ 11 kg = Mass