Calculate the number of Frenkel defects per cubic meter in zinc oxide at $939C.$ The energy for defect formation is $2\mathrm{.}51eV,$ while the density for ZnO is $5\mathrm{.}55\frac{g}{c}{m}^3$ at this temperature. The atomic weights of zinc and oxygen are $65\mathrm{.}41\frac{g}{m}ol$ and $16\mathrm{.}00\frac{g}{m}ol,$ respectively.

$N_{fr}=$ defects$/\frac{?}{m^3}($answer in E$)$