Assuming #x!=0 and y!=0 and x!=y#
#(x^3y - xy^3)/(xy^2-x^2y)#
#color(white)("xxxxxxxx")#extract common factors
#=((xy)(x^2-y^2))/((xy)(y-x))#
#color(white)("xxxxxxxx")# divide out common factors, factor numerator, & negate denominator
#= ((x+y)(x-y))/((-1)(x-y))#
#color(white)("xxxxxxxx")#divide out common factor
#= -(x+y)#