A charge +q is placed at the origin O, work done to take a charge Q from the point (a,0) to (0,b) is 

Potential at point A is ${\mathrm{V}}_{\mathrm{A}}=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\frac{\mathrm{q}}{\mathrm{a}}$
Potential at point B is ${\mathrm{V}}_{\mathrm{B}}=\frac{1}{4{\mathrm{\pi \epsilon }}_{\mathrm{o}}}\frac{\mathrm{q}}{\mathrm{b}}$
Work done in taking a charge Q from A to B is
$\mathrm{W}=\mathrm{Q}\left({\mathrm{V}}_{\mathrm{B}}-{\mathrm{V}}_{\mathrm{A}}\right)=\frac{\mathrm{Qq}}{4{\mathrm{\pi \epsilon }}_0}\left[\frac{1}{\mathrm{b}}-\frac{1}{\mathrm{a}}\right]=\frac{\mathrm{Qq}}{4{\mathrm{\pi \epsilon }}_0}\left[\frac{\mathrm{a}-\mathrm{b}}{\mathrm{ab}}\right]$