need help on a subwoofer cable?

My yamaha reciever has no speaker wire outs for the sub, just an RCA jack. is there any cable that goes from an RCA to speaker wire?
Use the RCA input on the sub. If it doesn't have one, you go from the speaker jacks on the receiver to the speaker in jacks on the sub and from the speaker out jacks on the sub to your speakers.
You need to run the rca-out of the receiver into the rca input of a powered subwoofer.
What kind of sub is it? No amp/receiver has a speaker out. If it is a high pass speaker connection, then you just connect the sub speaker connection to the same place as your regular speaker wires. It is OK to combine the two wires on the same connection. The sub is designed that way.
Speaker jacks are outputs.