x[t_, s_] = (s^2)
y[t_, s_] = 2  t + s
u[t_, s_] = 2  t + s
g1 = 
      ParametricPlot3D[{x[t, s], y[t, s], u[t, s]}, {s, -1, 
          1}, {t, -1, 1}, 
        PlotRange -> {{-1, 1}, {-1, 1}, {-1, 1}}, 
        ViewPoint -> {-2, -2, 0}]
x[t_, s_] = ((8  s)/(1 + 4  (s^2))) t + (s^2)
y[t_, s_] = ((2 - 8  (s^2))/(1 + 4  (s^2))) t + s
u[t_, s_] = 2  t + s
g2 = 
      ParametricPlot3D[{x[t, s], y[t, s], u[t, s]}, {s, -1, 
          1}, {t, -1, 1}, 
        PlotRange -> {{-1, 1}, {-1, 1}, {-1, 1}}, 
        ViewPoint -> {-2, -2, 0}]
Show[g1, g2]
u[x_, y_] = y
Plot3D[u[x, y], {x, -1, 1}, {y, -1, 1}]