Plot3D[1,{x,-3,3},{y,-3,3}]

Plot3D[{x^2},{x,-3,3},{y,-3,3}]

Plot3D[x^4 - 5x^3y - 5x^3 - y^5 -50x + 50y+30x y+ x^2y^2+3,{x,-2,2},{y,-2,2}]

Plot3D[-Sqrt[x^2 + y^2],{x,-3,3},{y,-3,3},PlotRange->{0,-3}]

Plot3D[x^2-2x + y^2 -4y +5,{x,-2,3},{y,-2,4},PlotRange->{0,10}]

Plot3D[x^2-y^2,{x,-3,3},{y,-3,3}]

Plot3D[8y^3+12x^2-24 x y ,{x,-1,2},{y,-1,2},PlotRange->{-10,10}]

ContourPlot[{8y^3+12x^2-24x y ==-3,8y^3+12x^2-24x y ==-2,8y^3+12x^2-24x y ==-1,8y^3+12x^2-24x y ==0,8y^3+12x^2-24x y ==1,8y^3+12x^2-24x y ==2},{x,-1,2},{y,-1,2},PlotLegends->"Expressions"]

f[x_,y_]:=x^2/2+3y^3+9y^2-3x y +9y  - 9x
Plot3D[f[x,y],{x,0,15},{y,-3,3},PlotRange->{-50,0}]
ContourPlot[{f[x,y] == -50, f[x,y]==-40,f[x,y]==-30,f[x,y]==-20,f[x,y]==-10,f[x,y]==0},{x,0,15},{y,-3,3},PlotLegends->"Expressions"]

ContourPlot[{x^2+y^2==4,x+y==-3,x+y==-2,x+y==-1,x+y==0,x+y==1,x+y==2,x+y==3},{x,-3,3},{y,-3,3},PlotLegends->"Expressions"]

ContourPlot[{x^2+y^2==4,3x+y==-6,3x+y==-4,3x+y==-2,3x+y==0,3x+y==2,3x+y==4,3x+y==6},{x,-3,3},{y,-3,3},PlotLegends->"Expressions"]

ContourPlot3D[50x + 30 y + 10 z ==18000,{x,0,18000/50},{y,0,18000/30},{z,0,1800}]

f[x_,y_]:=(x-1)^2+(y-2)^2
list=Join[{x^2+y^2==45},Table[f[x,y]==c^2,{c,1,9}]];
stylelist=Join[{{Black,Thickness[.015]}},Table[Automatic,{c,1,9}]];
ContourPlot3D[{f[x,y]==z,x^2+y^2==45},{x,-7,7},{y,-7,7},{z,0,50}];
ContourPlot[list,{x,-7,7},{y,-7,7},ContourStyle->stylelist,PlotLegends->"Expressions"]

f[x_,y_]:=x^2y + 3y^2 -y
list = Join[{x^2 + y^2 ==10},Table[f[x,y]==c,{c,0,30,5}]];
stylelist=Join[{{Black,Thickness[.015]}},Table[Automatic,{c,-10,10,2}]];
ContourPlot[list,{x,-4,4},{y,-4,4},ContourStyle->stylelist,PlotLegends->"Expressions"]