computer networks – Calculating minimum frame size

I’m actually quite stuck on a question I have for an assignment…I’d appreciate any help with this:

Consider a CSMA/CD network with maximum cable length of 8km and where the ratio of propagation speed to bandwidth (i.e. propagation speed/bandwidth) is 10 meters per bit. What is the minimum frame size in bytes?

I know that frame size, S >= 2BL where B is bandwidth, and cable length is L but I can’t quite get it done.