Do u only keep change flip when adding?

No.

Step-by-step explanation:

If I'm correct, you 'KCF' when dividing as well.

You use 'KCF whenever you're diving *and* adding.

e.g.

For this, you're going to 'keep' the first fraction:

(so nothing happens right now)

You then 'change' the division sign (/ or ÷) with a multiplication sign (× or ·)

Now, you are going to 'flip' the second fraction.

This is equal to 18/12 and can be simplified to 3/2