I didn't understand too much from hasenbraten's videos of mathematical concepts of EV(maybe that's why the AI EV line on PT3 has minus in front of it :) ), I understand much better when someones explains to me through examples.

Let's take a random hand on NL5 :
I am Button with K :spade: T :spade: and the UTG raised first, I called.

On the flop the pot is at 2$.
The community cards are 4 :spade: 5 :spade: 9 :diamond: .If I give him a tight range a have almost 50% equity.He has 4 $ left.So if he makes a pot size bet , he gives me 2:1 pot odds, and the flush draw has 4:1 odds to complete.
Also if I use implied odds, I must pay 2$ in order to win 6$ that means 3:1 implied odds.

->My question is : if I have 50% equity I need at least 2:1 pot odds to be +EV?
Or I calculate with implied odds and if I have less equity then 33%( 3:1 implied odds) it will be -EV?

Also let's speculate that he has three-of-a-kind,nines,the equity is 25 %.
So if he bets potsize, and with implieds I have 3:1 but the equity is less then 33%.So a call here is -EV?