Like this perhaps?
p1 = Plot3D[{-0.065306 x^(2) + 0.228571 x + 16.75}, {x, 0, 3.5}, {y, 0, 20}] p2 = Plot3D[{-x + 20.25}, {x, 3.5, 4.4}, {y, 0, 20}] Show[p1, p2, PlotRange -> All]
Yes, that worked out very well, thank you so much!:-)
