Plot[{Sqrt[x],2 + 1/4(x-4)},{x,0,100},PlotStyle->Thick]

ContourPlot[{y==CubeRoot[x],x==0},{x,-2,2},{y,-2,2},ContourStyle->{{Thickness[.01]},{Thick,Dashed}}]
2.25*2.25

1.75*1.75

f[x_]:=Piecewise[{{1,x≥0},{0,x<0}}]
ContourPlot[{y==f[x],x==0},{x,-2,2},{y,-2,2},ContourStyle->{{Thickness[.01]},{Thick,Dashed}}]

ContourPlot[{y==Abs[x],y==x,y==-x,y==0},{x,-2,2},{y,-2,2},ContourStyle->{{Thickness[.01]},{Thick,Dashed},{Thick,Dashed},{Thick,Dashed}}]

ContourPlot[{y==CubeRoot[x^2],x==0},{x,-2,2},{y,-2,2},ContourStyle->{{Thickness[.01]},{Thick,Dashed}}]

ContourPlot[x^2+y^2==25,{x,-5,5},{y,-5,5},ContourStyle->{Black},Axes->Thick,Ticks->{{},{}}]

ContourPlot[x^3+y^3==6x*y,{x,-5,5},{y,-5,5},ContourStyle->{Black},Axes->Thick,Ticks->{{},{}}]

ContourPlot[y Cos[x]==1 + Sin[x y],{x,-10,10},{y,-10,10},ContourStyle->{Black},Axes->Thick,Ticks->{{},{}}]