To solve this problem we will apply the laws of gaus that relate the Electric Flow as the charge of the object on the permittivity constant of free space. Mathematically this is

Rearranging to find the charge,

Here
Q = Charge
 Electric Flux
 Permittivity of free space
The total flux would be



Replacing we have that,



Therefore the charge Q inside a rectangular box is -21.24nC