Needs["PlotLegends`"];
m1=0.5;f1[x_]:=m1*x;
m2=1;f2[x_]:=m2*x;
m3=2;f3[x_]:=m3*x;
m4=-3;f4[x_]:=m4*x;
m5=-1;f5[x_]:=m5*x;

Plot[
{ f1[x],f2[x],f3[x],f4[x],f5[x]},{x,-10,10},PlotLabel->"graphs with various slope",
AxesLabel->{"x axis","y axis "},
PlotRange->{ {-5,5},{-20,20}},
PlotStyle->{
{Black,Thick},{Red,Dotted},Blue,Purple,Orange
},
PlotLegend->{"f1", "f2","f3","f4","f5"}
]

g1[x_]:=Sqrt[-x];
g2[x_]:=Sqrt[x];
g3[x_]:=-Sqrt[x];

Plot[
{g1[x],g2[x],g3[x]},{x,-4,4},PlotLabel->"graphs",
AxesLabel->{"x axis","y axis "},
PlotRange->{ {-5,5},Automatic},
PlotStyle->{Yellow,Blue,Purple}
]