How do I find a concentration from a sample tested on a spectrophotometer?
The question being asked is, how many uL of your solution do you need to withdraw in order for that volume to contain 100 ng of the RNA.  If there are 2 ug/uL in the RNA solution, that is equivalent to 2000 ngm/uL.  So if you want 100 ng of RNA, you will need to withdraw 
$$\ce{\frac{2000 ngm}{1 uL}~=~ \frac{100 ngm}{x ~uL}}$$
solving for x yields an answer of 1/20 uL or 0.05 uL.  0.05 uL of the RNA solution will contain 100 ng of the RNA.