Answer
$U_ b = 7.3 \times 10^{-8} \,\text{J} $
Work Step by Step
The work done is the difference between the two potential energies at the different points and we could get the potential energy at point $b$ by using this concept by
\begin{gather*}
W = U_a - U_b \\
U_b = U_a - W\\
U_b = + 5.4 \times 10^{-8} \,\text{J} - (- 1.9 \times 10^{-8} \,\text{J} )\\
\boxed{U_b = 7.3 \times 10^{-8} \,\text{J} }
\end{gather*}
