Always try to draw it first. If then when y=0, you have the line z=1-x in the z-x plane and if x=0, you have z=1-y in the z-x plane. Now just connect the lines in the x-y plane between y=1 and x=1. Try and draw those three lines in an x-y-z coordinate axis on paper then try to understand why the integral is (I believe):