v² + 12v + 20 = (v + 10) (v + ?)
First, we are going to try some low numbers and use guess and check.
Let's try 6, it is a small number, and seems good.
(v+10)(v+6)
v²+6v+10v+60
v²+16v+60
6 is too large a number, so we'll use 4.
When ?=4:
(v+10)(v+4)
v²+4v+10v+40
v²+14v+40
Once again, 4 is too large, so let's try 2.
When ?=2:
(v+10)(v+2)
v²+2v+10v+20
v²+12v+20
SO, YOUR ANSWER IS ?=2
~Hope I helped!~
(Also, guessing and checking may take some time, but it is the easiest way for me to do these types of questions.)
