$ I_1=\frac{9}{4} \times \frac{4}{16}=\frac{9}{16} $
$ V_A-V_B=I_1 \times 8=\frac{9}{16} \times 8=\frac{9}{2} V $
$ \therefore U=\frac{1}{2} \times 4 \times \frac{81}{4} \mu \mathrm{J} $
$ \therefore U=\frac{81}{2} \mu \mathrm{J} $
$ \therefore \mathrm{X}=81$





Reason: An electron has a negative charge.