As the plates are square so length (l)= widith (w).
here, A=lw=l2→l=√A
There are two capacitors - one is air filled and other is dielectric filled and they are in parallel.
For air filled capacitor: area A1=(l−x)l=(√A−x)√A=(A−√Ax) and
Capacitance, C1=A1ϵ0d=(A−√Ax)ϵ0d
For dielectric field capacitor: area A2=lx=√Ax and
Capacitance , C2=A2ϵrϵ0d=√Axϵrϵ0d
The net capacitance is C=C1+C2=(A−√Ax)ϵ0d+√Axϵrϵ0d=ϵ0d(A−√Ax+ϵr√Ax)