A small metal sphere carries -7.5 nC. Treat it as a point charge in air…

Step 1. The product $kq$. $kq = 9.0 \times 10^{9}\ \text{N m}^2/\text{C}^2 \times (-7.5 \times 10^{-9}\ \text{C}) = -67.5\ \text{V m}$.

Step 2. At $12$ cm. $V = \dfrac{kq}{r} = \dfrac{-67.5\ \text{V m}}{0.12\ \text{m}} = -562.5\ \text{V}$.

Step 3. At $36$ cm. $V = \dfrac{-67.5\ \text{V m}}{0.36\ \text{m}} = -187.5\ \text{V}$.

Step 4. The work. The charge moves from $-187.5$ V to $-562.5$ V, so $\Delta V = -375$ V. $W = q\,\Delta V = 2.0 \times 10^{-9}\ \text{C} \times (-375\ \text{V}) = -7.5 \times 10^{-7}\ \text{J}$, that is $-0.75\ \mu\text{J}$.