pts={{0,0,0},{0,1,2},{1,1,1},{1,0,5}};
makePlane:=Module[​​{g,x,y},​​g[x_,y_]=Fit[pts,{1,x,y},{x,y}];​​findPt[g]​​]
findPt[pl_]:=Module[​​{},​​pl[1,2]​​]
makePlane
1.5