Show[RootLocusPlot[k (100 s (s+10))/((s+25) (s-10) (s^2-225)),{k,0,30},PlotRange->All,Frame->True,AspectRatio->1,PlotStyle->(Directive[AbsoluteThickness[8],ColorData[17,#]]&/@{2,7,4,9}),PoleZeroMarkers->{Style["\[Cross]",White,16],Style["●",White],"ParameterValues"->10},Method->{"AxesInFront"->False}],Method->{"AxesInFront"->False}]