NMaximize[{X1,{0≤X2≤2,0≤(X1-X2)/Sqrt[2]≤1,0≤(X1-X2)/Sqrt[2]≤1.5}},{X1,X2}]

Solve[(X1-X2)/Sqrt[2]==1,X2]

Solve[(X1-X2)/Sqrt[2]==1.5,X2]

g1=((-2+(Sqrt[2]) X1)/(Sqrt[2]))

g2=2

g3=-1.4142135623730951` (1.5` -0.7071067811865475` X1)



Plot[{g1,g2,g3,g4},{X1,-3.0,6.5},PlotRange->{{-3.0,6.5},{-3.0,3.0}},Filling->Bottom]
Show[%, Frame->True,GridLines->Automatic,FrameLabel->{X1,X2},LabelStyle->Directive[Red]]