The electric field in a certain region is acting radially outward and is given by E = Ar. A charge contained in a sphere of radius 'a' centred at the origin of the field, will be given by :

Flux linked with sphere = E→.d s→Since electric field is radial. It is always perpendicular to the surface.So  ϕ=Ar.4πr2     ϕ=Aa4πr2    as r=a ϕ=A 4πa3Now according to Gauss's lawϕ=qinε0⇒qin=ϕ.ε0So, qin=A4πa3ε0