@lgbasallote that was a proof by contradiction, not by contrapositive.
In a proof by contradiction (reductio ad absurdum) you begin by assuming the opposite of the statement that you hope to prove. If you hope to prove that sqrt(2) is irrational, you begin with the assumption that it's rational. If you hope to prove that objects fall down, then you begin with the assumption that they fall up. If you hope to prove that God exists, you begin with the assumption that God does not exist. Once you've made that assumption, you follow logic to some absurd, obviously false conclusion. As long as all of your logic is sound, you can conclude that your assumption was false. Since you assumed the opposite of what you wanted to prove, then you've proven your case by disproving the opposite of it.
Proof by contrapositive is a little different. You still do a direct proof of some sort. You don't assume the opposite of your statement or anything, but instead begin with a normal assumption. However, what is different is that instead of proving the statement itself, you prove the contrapositive of the statement, which is logically equivalent.
An example of this would be;
I want to prove the statement "If a number is even, then it can be divided by 2."
Instead, I could choose to prove the logically equivalent contrapositive, which is, "If a number cannot be divided by 2, then it is odd."