Decompose the terms of #x^(3a)y^a - y^(3a)x^a#
as #= x^(2a)*x^a*y^a - x^a*y^a*y^(2a)# #color(white)("XXXX")#now we can see an obvious common term #x^ay^a
#=(x^ay^a)(x^(2a)-y^(2a))#
#=(x^ay^a)((x^a)^2 -(y^a)^2)# #color(white)("XXXX")#the second term is the difference of squares, so #=(x^ay^a)((x^a+y^a)(x^a-y^a)#