#### Answer

$7.99\ g/cm^3$

#### Work Step by Step

If the diameter is 9.40 mm, the radius (half of it) is $4.70\ mm=0.470\ cm$.
The volume of the sphere is: $V=\frac43\pi r^3=0.435\ cm^3$
The given mass is 3.475 g, so the density is:
$\rho=m/V=7.99\ g/cm^3$