I read in a Sefer about a situation where a person uses the facilities before making a Bracha Achronah . My question is , do you make an Asher Yotzar first and then the Bracha Achronah or vice versa ?
Since the bathroom usage had just occurred and needs to be handled, and a bracha should be made as close as possible to the action that requires it, logic would indicate that it should be done first. Additionally, one becomes obligated immediatel upon using the bathroom no matter how minimal the usage. One is not necessarily obligated to say the bracha acharona as one could still continue eating. This is based on logic only. However, as seen below, the logic is not completely true. It would depend on whether of not one intends to eat or drink more.
Halachipedia states :
Asher Yatzar vs. Other Berachot
If one uses the bathroom and additionally needs to recite boreh nefashot on food that he ate, asher yatzar should be recited first. 
If one uses the bathroom and then needs to recite asher yatzar as well as birkat hamazon or al hamichya, birkat hamazon or al hamichya take precedence. 
One may recite asher yatzar after the beracha of hamapil if he uses the bathroom.
If one left the bathroom and then heard thunder or saw lightning before reciting asher yatzar, he should recite the beracha on the thunder or lightning because the time for those passes quickly. 
 Mishna Brura 7:2 says this based on tadir visheayno tadir, tadir kodem. Yalkut Yosef Kitzur Shulchan Aruch 7:9
 Yalkut Yosef Kitzur Shulchan Aruch 7:9
 Halichot Shlomo 20:44, Sh"t Yechave data 4:21, Sh"t Tzitz Eliezer 7:27, Sh"t beer Moshe 1:63
Mishna Brura 227:12