\(V _{ C } =\frac{ C _{1} V _{1}+ C _{2} V _{2}}{ C _{1}+ C _{2}}\)
\(=\frac{ C \times 100+ C \times 0}{ C + C }\)
\(=50\,Volt\)
Electrostatic energy stored
\(=2 \times \frac{1}{2} CV^{2}=CV^{2}\)
\(=900 \times 10^{-12} \times 50 \times 50\)
\(=225 \times 10^{-8}\,J\)
\(=2.25 \times 10^{-6}\,J\)