\(\frac{1}{2} mv ^{2}-\frac{1}{2} mu ^{2}=-1.6-10^{-19} \times 0.4\)
\(\frac{1}{2} 9 \times 10^{-31}\left( v ^{2}- u ^{2}\right)=-0.64 \times 10^{-19}\)
\(u ^{2}- v ^{2}=\frac{2 \times 0.64 \times 10^{12}}{9}\)
\(v ^{2}=\left(36-\frac{128}{9}\right) \times 10^{10}\)
\(v =\frac{14}{3} \times 10^{5} \,m / s\)
\(x =14\)