3x^2+6x+3
=3(x^2+2x+1)
=3(x+1)^2
-a^2-4b^2+4ab
=-(a^2+4b^2-4ab)
=-(a-2b)^2
(p^2+q^2)^2-4p^2q^2
=[(p^2+q^2)-2pq][(p^2+q^2)+2pq]
=[(p-q)^2][(p+q)]^2
=(p-q)^2(p+q)^2