Hi I really have no idea which catergory this question falls into, especially so when I don't even know how to start off the question..

If Person A is 20Km north of Person B at 5pm. Person A walks south at a rate of 7Km/h and Person B walks west at a rate of 9Km/h. What time do these two peopole stop approaching each other and instead, drift further apart?

How do I start and what formula should I use? I'm really clueless. Is it under bearings..?

Thank you so much, really!