2p^2+3p+1=0
You need 2 numbers that mult to 2*1=2 and add to 3 That will be 2 and 1
now replace 3p with 2p+1p
$$\\2p^2+3p+1=0\\\\
2p^2+2p+1p+1=0\\\\
$now factor in pairs$\\\\
2p(p+1)+1(p+1)=0\\\\
(2p+1)(p+1)=0\\\\
2p+1=0\;\;or\;\;p+1=0\\\\$$
And you can finish it :)