g1=1/2 *(-4+X1)

g2=1

g3=-2+X1

g4=-1

Plot[{g1,g2,g3,g4},{X1,-3.5,6.5},PlotRange->{{-3.5,6.5},{-3.5,3.5}},Filling->Bottom]
Show[%, Frame->True,GridLines->Automatic,FrameLabel->{X1,X2},LabelStyle->Directive[Red]]