x[t_, s_] := -2 t + 3  s
y[
        t_, s_] := t + 2
u[t_, s_] := 
      t + 4 - 3  s
ParametricPlot3D[{x[t, s], 
          y[t, s], u[t, s]}, {s, -100, 100}, {t, -100, 100}, 
        AspectRatio -> 1,  
        PlotRange -> {{-200, 200}, {-200, 200}, {-200, 
              200}}];

Plot3D[-y - x + 6, {x, 10, -10}, {y, 10, -10}, 
      ViewPoint -> {10, -10, -10}]