RUNNING POWERED SPEAKERS FROM AN IMAC

Hello all:
What am I missing here? How would I go about using my powered speakers, Kantos i pair 5's with my iMac? They have s video and rca jacks etc. but the iMac has only a headphone jack and usb ports. Is there a way to do this? I no longer need these speakers in a home office and would love to repurpose them in the computer room.
Thanks for any suggestions!