Yeh, substituting. Before I knew enough to be able to do this sort of thing as close to properly as I do, I used to just play around with substitution as it was the only way I could work out how to do it with common sense alone. Sometimes it could take a while. For years I developed my own common sense coping strategies for maths as I didn't know the correct methods. I very much doubt I came up with anything new but I did come up with them myself, they were never very quick though. You may see that I do some things a little bit unconventionally.
3x2+36=24x Subtract 24x from both sides;
3x2-24x+36=0 Not sure if I needed to put the "-24x" in this position but it normally sits here as B in the formula so thought it best? Then I factorised;
3(x2-8x+12)=0 Then I divided both sides by 3 which seemed correct for the left hand side but seemed like a bit of a cheat for the right hand side as $${\frac{{\mathtt{0}}}{{\mathtt{3}}}}$$ stays as 0, but it seems to work;
x2-8x+12=0 Then I factorised again;
(x-6)(x-2)=0 So x=6 or x=2
I have expanded the brackets and substituted the numbers and all seems to check ok. This really does help to make sense of factorising, it gives it a purpose.
I'm still never satisfied though to come up with an answer of it could be this or it could be that, I've always been used to there only really being one correct answer in maths, different ways of getting there but only one correct answer.