3-y x-3
--------------- + -----------
y^{2} - xy xy-x^{2}

( 3 - y ) / ( y² - xy ) + ( x - 3 ) / ( xy - x² ) = ( 3 - y ) / ( y( y - x )) + ( x - 3 ) / 
/ ( x( y - x )) = ( x( 3 - y ) + y( x - 3 )) / ( xy( y - x )) = ( 3x - xy + yx - 3y ) /
/ ( xy ( y - x )) = ( - 3( y - x )) / ( xy( y - x )) = ( - 3 ) / ( xy )