Potential at a point due to conducting sphere and sheet

[olivia rogers]

There's this question, A conducting sphere of radius R and charge Q is placed near a uniformly charged non conducting infinitely large thin plate having surface charge density σ. The potential at point A on sphere due to charge on sphere is? I have uploaded the picture.
Now I know the net field inside a conductor is zero. So the sum of Electric field due to sphere and field due to sheet should be zero. So both are equal in magnitude. So the net potential should be kQ\R + potential due to sheet. How do i calculate potential due to sheet??

 

[Wee-Lamm]

http://www.phys.uri.edu/gerhard/PHY204/tsl92.pdf