acc to your question point charge is inside the spherical shell.
this +q charge will induce and create -q charge in the inner portion of spherical shell.
why because to the electric field to be zero inside the shell net charge enclose by gaussian surface must be zero.
but as net charge on shell is zero therefore +q charge will be created on the outermost part of shell.
to find the potential at the centre of shell you have to take into account three charge species .
1-the point charge itself.
2-inner shell with -q charge
3-outer part of shell with +q charge.
hope it helps...