The three charges are placed at the corners of an equilateral triangle as shown in the following figure. The potential energies of the three charges are given as
U1=
1
4πε0
×
3nC×6nC
0.1
=
1
4πε0
×
18×10−18
0.1
U2=
1
4πε0
×
6nC×9nC
0.1
=
1
4πε0
×
54×10−18
0.1
U3=
1
4πε0
×
9nC×3nC
0.1
=
1
4πε0
27×10−18
0.1
Therefore, potential energy of the system is given as