Let V0  be the potential at the origin in an electric field E→=Exi^+Eyj^. The potential at the point (x,y)  is

Ex=−∂V∂x Ey=−∂V∂yIf y is constant (=0),∫V0VAdV=−∫0xExdxOr VA−V0=−Exx.Where x is constant (=x),∫VAVBdV=−∫0yEydyOr VB−VA=−Eydy .Adding, VB−V0=−xEx−yEyOr VB=V0−xEx−yEy .