i know i can make dual graphics with a10-7850 and r7 250 but i don't know if it's possibile with r7 250 x.
what's the differences between r7 250 and r7 250x?
No you can't, and you should stop talking to whoever told you Dual Graphics was a good idea. If you want to play Facebook games, stick with just the integrated GPU. If you want to play actual games, get a 270X for $150, or the 280X for $200.