Answer
$\sigma=12\times 10^{-9} \frac{C}{m^2}$
Work Step by Step
We know that the surface charge density can be calculated as:
$\sigma=\frac{q}{A}$
For the sphere with area $A=4\pi r^2$:
$\sigma=\frac{3.3\times 10^{-9}}{4(3.1416) (0.15)^2}$
$\sigma=1.17\times 10^{-8}$
$\sigma=11.7\times 10^{-1}\times 10^{-8}$
$\sigma=12\times 10^{-9}$
$\sigma=12n\frac{C}{m^2}$