factor the polynomial

y2 + 6xy + 9x2      This is a perfect square trinomial....the key to factoring these is to take the square root of the first and last terms and separate them by the sign on the middle term....then just square the whole thing...

So we have.....

(y + 3x)

Now...just square the whole thing

(y + 3x)²

And there you are  !!!