0 = x2 + 6x + 9 factor by grouping explain

To factorise by grouping would require grouping the terms into the form x^2 + 2a + a^2 = (x-a) ^2. In which case, the a term would be 3, as 2a=6, a=3. Hence the factorised form would be (x-3) ^2=0 which means that x-3=0 and hence x=3.