Electric flux is a concept used in the field of electromagnetism. Every electric field creates an electric flux that is proportional to the electric field lines that go through a given area.
Electric flux over an area S is equal to the integral of E*dA over the entire surface area. Electric flux is expressed in the units N*m^2/C
Here, the electric field is 10 N/C. As the electric field is constant we can calculate the electric flux through the top face and the left face by multiplying the electric field by the area of two faces.
The electric flux through the top face is 10*0.01^2 = 10^-3 Nm^2/C
Similarly, the electric flux through the left face is also 10^-3 Nm^2/C