I really need help solving this problem!!?!

I really need help solving this problem!!?

When two people shook each others hand, so that was 1 handshake. When three people shook hands, there were three handshakes. When four people shook hands, there were ??? handshakes. Find the number of handshakes for 4 people, then write a quadratic function that models this situation.

6 days ago
i could figure out the answer really easily but i really need to the the quadratic formula!!

Two.. (put them in pairs.. they shake hands with each other)