Hard time understanding epicenter question

My teacher made this problem up and I'm having a hard time understanding what I'm supposed to do. We haven't worked with earthquake epicenters at all so I'm confused. The last thing we worked with was linear programming. Anyway here's the problem. Thanks for any help!

If you have ever experienced an earthquake, you realize that it causes a primary wave and also a secondary wave to be created. The primary wave (a logitudinal wave) travels faster, so you feel it first. Shortly afterward you will feel the secondary wave (a transverse wave), which we refer to as an aftershock. For our minor earthquake, the primary wave traveled at 6km/s and the secondary wave traveled at 3km/s. In the late afternoon we felt a small earthquake and 4 seconds later came the aftershock. **How far away from the epicenter of the earthquake were we located?**

__Her hint to me was__: *Three equations, three unknowns*.