A Slingbox attaches to output of Cable Box (or whatever) and SENDS the Hi-Def A/V Signal through an Internet (Internal and/or External) Connection to a remote COMPUTER for display.....not cheap and not what you need.
IF you followed Dr Don's links above, you would find out you have TWO OPTIONS:
1) Pair of WIRELESS HDMI EXTENDERS
, such as the following example: https://www.amazon.com/IOGEAR-GW3DHD...Bextender+-cat
Note that conformance with HDMI v1.3 (or later) provides compatibility with all HDMI HIGH SPEED Cables and hence "should" also work with v1.4 thru v2.0 HDMI Interfaces.
2) Pair of POWERLINE HDMI EXTENDERS
, such as the following example: https://www.amazon.com/HDbitT-Extend...=1&*entries*=0
[Connects via RF Signal running on top of House 120 VAC Wiring]
Since the Wireless Wi-Fi signal would only have to go thru the Living Room's Floor, there shouldn't be ANY connectivity problems.....however, the (more expensive) Powerline solution in some cases MIGHT have connectivity problems since each location probably has a separate 120 VAC circuit running all the way back to the Circuit Breaker Box....and all of the EMI/RFI Filters in various Electrical/Electronic/Surge Protectors on each Circuit can attenuate the Powerline RF signal.
So I advise to use a Pair of WIRELESS HDMI EXTENDERS....and unless you can get a COMCAST Cable Box with R/C via RF Link, you'll also want a WIRELESS IR EXTENDER so you can remotely control the Cable Box in the Living Room:
And since COMCAST Cable Box only had ONE HDMI Output Port, you'll want an inexpensive 2-PORT HDMI SPLITTER, with has TWO HDMI Output Ports [lots to chose from]: