Ta Temperature Primer What Is The Relation Between Ta And Tm Of The Primers In PCR Reactions? ?

What is the relation between Ta and Tm of the primers in PCR reactions? ? - ta temperature primer

I read a little reading online and Ta should be 5 degrees below MC?
Tm is the melting temperature of DNA take place?
What does this formula: Tm = 4 (G + C) + 2 (A + T) OC? is G, C, A, T, the number of bases in the primer?
Thank you, I need help urgently