Answer
$W_{ext}=0.512J$
Work Step by Step
$W_{ext}=q(V_b-V_a)=(qkQ)(\frac{1}{r_{1f}}+\frac{1}{r_{2f}}-\frac{1}{r_{1i}}-\frac{1}{r_{2i}})=(0.50\mu C )\times(8.99\times10^9\frac{N\cdot m^2}{C^2})(35\mu C)(\frac{1}{0.11m}+\frac{1}{0.35m}-\frac{1}{0.23m}-\frac{1}{0.23m})=0.512J$