easy situation :

[spoohunter]

Turn I have 67s. Board is 3,5,9,J with two spades. I have 15 outs to win.

Pot is $31.

I bet $31. Opponent raises me $100 more, all in.

My equity is $64 so i fold right? I'm unsure, thank you, please clear up.

I have opponent covered.

[yellowjack]

Your equity in the pot depends on what your opponent has. In the most extreme cases, it can be as low as 13.64% or as high as 47.73%.

I'd put villain on two pair or a set, so this leaves your equity at 30%. So overall there's $162 in the pot and $69 to you. 69/162 = 42.5%. Fold. This isn't a surprising result for you, but it's closer than I had expected. This is of course assuming a ring game where losing the money you have is of no consequence (if it was a significant portion of your bankroll you'd wait for a better spot to reduce variance and finda better spot).

[KJL]

Where did you get $64 from? You have about a 33% chance to win the hand and there is $162 in the pot. So in order to call the bet has to be less than $53.