Answer
After the spheres are separated, each sphere will have a charge of $12.0~\mu C$
Work Step by Step
When the two spheres are brought in contact, the charges will rearrange themselves so there is an equal charge on each sphere (since the two spheres are identical). Therefore, after the spheres are separated, each sphere will have half of the total initial charge.
$\frac{6.0~\mu C+ 18.0~\mu C}{2} = 12.0~\mu C$
After the spheres are separated, each sphere will have a charge of $12.0~\mu C$
