Proofs are not magic. They simply require you to think logically. It is likely you have not encountered such problems before. That's what geometry is all about. You must learn to think the problem through, using the information at hand and other things you know.
1) I need to prove {blah}
2) In order for {blah}, this would have to be equivalent to that.
3) Those two are equivalent if they are parallel
4) They are parallel if that other thing is perpendicular to both
5) Aha! That's what they told us in the problem statement.
6) You're done.
There is not a formula. It requires you to think on it and figure it out. Have you a specific problem you are working on?
