spoocter:
How do i make it like this?
What you're doing is fine. The difference is that a++ modifies a. The way abs() is implemented, it would case a++ to happen twice, which would mean a is one bigger than it should be. The subtraction you're doing is fine, though.
If you really want to know:
void loop(){
pot = analogRead(A1);
// Do your stuff here
int potdiff = pot - pot_before;
potdiff = abs(potdiff);
if (potdiff > 2) {
//do what you want to do here if the difference is bigger than 2
}
pot_before = pot;
}