Area of each surfaces a=12cm2=10−4m2.
As the field is in the direction of x-axis so only surfaces x=0 and x=1 will contribute the flux.
For surface x=0,ϕ0=E.a=5×103×x2.a=5×103×02.a=0
and for x=1,ϕ1=E.a=5×103×x2.a=5×103×12×10−4=0.25
By Gauss's law, ϕ1=qenϵ0
⇒qen=0.25×8.85×10−12=2.21×10−12C