( x" + y" )( x" - 4 + y" ) = -4
( x" + y" )( x" + y" - 4 ) + 4 = 0
( x" + y" )" - 4( x" + y" ) + 4 = 0
( x" + y" - 2 )" = 0
x" + y" = 2
( x" + y" )( x" - 4 + y" ) = -4
( x" + y" )( x" + y" - 4 ) + 4 = 0
( x" + y" )" - 4( x" + y" ) + 4 = 0
( x" + y" - 2 )" = 0
x" + y" = 2