The charge distribution is shown in figure. Due to q1, the charge induction is non - uniform but net induced charge due to q1 is zero. At point A field is zero and potential is equal to the potential at center.
VA=VC=
1
4πε0
q
R
+
1
4πε0
q1
a
At point B, field is due to charge q only EB=
1
4πε0
q
y2
At point B, potential is due to charge q at center of cavity, −q induced on inner surface of cavity in addition to the potential at the center. ∴VB=