pts={{0,0,0},{0,1,2},{1,1,1},{1,0,5}};
makePlane:=Module[​​{plane},​​plane=Fit[pts,{1,x,y},{x,y}];​​f=Function[{x,y},plane];​​findPt[f]​​]
findPt[pl_]:=Module[​​{},​​pl[1,2]​​]
makePlane
1.5+2.x-1.y
f[1,2]
1.5
findPt[f]
1.5+2.x-1.y