I think you misstated the question.
(x+3y)/(x2+2xy+y2)+(x-y)/(x2+4xy+3y2).
Note that x2+2xy+y2=(x+y)2 and x2+4xy+3y2=(x+3y)(x+y).
The LCD of the denominators is (x+3y)(x+y)2.
The expression becomes:
[(x+3y)(x+3y)+(x-y)(x+y)]/[(x+3y)(x+y)2]=
(x2+6xy+9y2+x2-y2)/[(x+3y)(x+y)2]=
(2x2+6xy+8y2)/[(x+3y)(x+y)2]=
2(x2+3xy+4y2)/[(x+3y)(x+y)2].