<< Graphics`ContourPlot3D`
a = 
      Plot3D[0.5  x + 2  y, {x, -5, 5}, {y, -5, 5}, 
        AxesLabel -> {"x", "y", "u"}]
b = 
      ParametricPlot3D[{2  t + s, 0.5  t + s, 2  t + 2.5  s}, {t, -1, 
          1}, {s, -1, 1}, 
        AxesLabel -> {"x", "y", "u"}]
c = 
      ParametricPlot3D[{s, s, 2.5  s}, {s, -3, 3}, 
        AxesLabel -> {"x", "y", "u"}]
d = 
      ContourPlot3D[(x - 1)*(y - 
                1) - (1/4) (((u - 2.5))^2), {x, -1, 1}, {y, -1, 
          1}, {u, -1.5, 4.5}, 
        AxesLabel -> {"x", "y", "u"}]
\(e = 
      ContourPlot3D[\((x + 1)\)*\((y + 
                1)\) - \(1\/4\) \((u + 2.5)\)\^2, {x, \(-2\), \(-1\)}, {y, \(-2\), \(-1\)}, {u, \(-4.5\), 0}, 
        AxesLabel -> {"x", "y", "u"}]\)
f = 
      ContourPlot3D[(x + 2)*(y + 
                2) - (1/4) (((u + 5))^2), {x, -1, 2}, {y, 2, 
          5}, {u, 0.5, 10}, 
        AxesLabel -> {"x", "y", "u"}]
Show[a, b, c, d, e, f]