Need to hook up a vcr/dvd to a TV which has only a cable line in jack no av jacks

You need something equivalent to Radio Shack Catalog #: 15-2526 RF Modulator. From their web site: "This RF Modulator converts the video and stereo audio from any A/V component, and most video game decks, into a coaxial cable output. All you do is connect your A/V component to the modulator's input jacks, then connect the modulator to your TV's antenna/cable input and select channel 3 or 4 operation. "

Video game retailers used to sell similar units, but since many late model TVs have AV inputs I'm not sure if they still carry them.