It's not clear to me how the receiver designers expect you to connect a
75 ohm coax, even though they have markings for it. I would get a 75
to 300 ohm transformer, which will take the coax on one end and give
you two 300 ohm leads similar to what you have coming off the AM
antenna. Then you can hook the AM and FM to the spots indicated for
the FM 300 ohm and AM inputs. The transformer is available at most
places they sell antennas stuff, like Radio Shack, for a few bucks.