By best I mean best for you. I posted this thread to find out different ways people use. I know what I've been taught is popular according to my teacher.
My way is:
write the first and last numbers. The middle number "10a" is what we will divide into two numbers.
Multiple 3 by 7= 21.
Our goal is similar to a normal trinomial without a leading coefficient more than one now. We need to find a number that when multiplied is 21 and added 10. Eventually, the numbers 3 and 7 will come up. Better to go in chronological order for those who do not know.
Now use grouping method.
Take the numbers 3a and 7... put them in (3a+7)
(a+1) (a+1) is similar to doing it with (3a+7)
This is how I do it. It might not make sense for you, but it does for me.
Now, what's your way?
The method you described above called: Vieta's Formulas -- from Wolfram MathWorld