grtck=Range[-5,5]

Manipulate[Plot[m x + b,{x,-6,6}, PlotStyle-> {Red,Thick},AxesStyle->Thick,BaseStyle->{14},AspectRatio->Automatic,
PlotRange->{{-6,6},{-6,6}},Ticks->{grtck,grtck},GridLines->{grtck,grtck},ImageSize->300],{m,-5,5},{b,-5,5}]