a=RegionPlot[And[x≤4000,x≥800,y≤1000,x+y≤4200,20x+15y≤50000],{x,0,3000},{y,0,1500},PlotPoints->60]

b=ContourPlot[5000 x + 2800 y,{x,0,3000},{y,0,1500},ContourShading->False,ContourLabels->True,Contours->16]

Show[a,b]

x0=50000/20

c=ContourPlot[5000 x + 2800 y==(2500)(5000),{x,0,3000},{y,0,1500}]

Show[a,b,c]

Plot3D[{10-(x^2)-(y^2),4},{x,-2,2},{y,-2,2},RegionFunction->Function[{x,y,z},x^2+y^2≤4],BoxRatios->Automatic,Boxed->False,MeshFunctions->(((#1)^2)+((#2)^2)&),PlotStyle->{{Opacity[0.2],LightGreen},{Opacity[0.2],LightGreen}},PlotPoints->60]

Plot3D[z=(x^2)-(y^2),{x,-2,2},{y,-2,2},MeshFunctions->(#3 &),RegionFunction->Function[{x,y,z},x^2+y^2≤4],PlotStyle->{Opacity[0.2],LightGreen},BoundaryStyle->Thick,PlotPoints->50]

ContourPlot[(x^2)-(y^2),{x,-2,2},{y,-2,2}]