f[x_, y_] := x^2 + y^2

gu = 
      Plot3D[f[x, y], {x, -10, 10}, {y, -10, 10}, 
        PlotRange -> {0, 120}]

(* nice,  ale xcem to vidiet krajsie,  
      cize to dam do kocky *) 
 (* 
      pouzijem tu vlastnost DisplayFunction -> Identity,  
      ktora zaezpeci,  
      aby sa to nenakreslilo (vysledny subor bude teda mensi) *)

gu = 
      Plot3D[f[x, y], {x, -10, 10}, {y, -10, 10}, 
        BoxRatios -> {1, 1, 1}, PlotRange -> {0, 120}, 
        DisplayFunction -> Identity]

go = 
      ParametricPlot3D[{x, 10 - x, z}, {x, 0, 10}, {z, 0, 120}, 
        BoxRatios -> {1, 1, 1}, 
        DisplayFunction -> Identity]

Show[gu, go, AxesLabel -> {"x", "y", "z"}, 
      DisplayFunction -> $DisplayFunction]

Show[gu, go, AxesLabel -> {"x", "y", "z"}, 
      DisplayFunction -> $DisplayFunction, 
      ViewPoint -> {1.978,  2.313,  1.479}]