I have a new RTH7500D programmable thermostat, On my old thermostat I have wire W2, Y, G, R, B, and X. I need to know where to connect the B and X wire.

The new t-stat should have a terminal designated "O/B" - connect wire previously used on "B" terminal. "C" on new t-stat is probably equivalent to "X" from old one. You will need to access setup menu and configure "O/B" terminal to power reversing circuit in heating mode.