Calculating the molarity of DNA in a cell
There is one particle of dsDNA in the cell. Divide by $N_\mathrm{A}$ to get the amount of substance, and divide by the volume to get the concentration matching the concentration given in the answer.
\begin{align}
  V_{\mathrm{cell}} 
    &amp;= 4 \cdot {(\pu{0.5 μm})}^{3}\\[0.5ex]
    &amp;= \pu{0.5 μm3}\\[0.5ex]
    &amp;= \pu{5E-16 L}\\[3.5ex]
  N_{\mathrm{A}}
    &amp;= \pu{6E23 1//mol}\\[3.5ex]
  n_{\mathrm{DNA}} 
    &amp;= \frac{1}{N_{\mathrm{A}}}\\[0.5ex]
    &amp;= \pu{1.7E-24 mol}\\[3.5ex]
  c_{\mathrm{DNA}}
    &amp;= \frac{n_{\mathrm{DNA}}}{V_{\mathrm{cell}}}\\[1.5ex]
    &amp;= \frac{\pu{1.7E-24 mol}}{\pu{5E-16 L}}\\[1.5ex]
    &amp;= \pu{3E-9 mol//L}
\end{align}