I need some guidance on this one please.

At midnight the Titanic is 100km due east of a big iceberg. The Titanic is sailing west at 12 km/hr and the iceberg is moving south at 10 km/hr. What time will the Titanic be closest to the iceberg and what will that distance be? Use the 2nd derivative test to prove the answer is the minimum.

Basically all I have at this point it my variables wrote down and converted to seconds and meters along with a picture. I don't know what to do next, please help. Thank you.