Question

If you travel 30 degrees south latitude 90 degrees longitude where would you be

Answer 1

OK, to start off, "North Latitude" means north of the equator, so this would be in the northern hemisphere. 
Then Longitude is stated as so many degrees East longitude of so many degrees West longitude. (These are East or West of Greenwich, England). 
Since you do not specify east of west: 
30 degrees North Latitude and 90 degrees West Longitude would be along the coast of the Gulf of Mexico - while 30 degrees North Latitude and 90 degrees East Longitude would be somewhere in southwestern China, probably in the Himalaya Mountains. 

Answer 2

According to NASA:

Center of the orthographic projection is at latitude 30 degrees S., longitude 90 degrees. The top half is dominated by the Tharsis Montes volcanoes, the large Alba Patera shield volcano, the dark Chryse basin, and a vast canyon system, Valles Marineris. In the central part, a prominent physiographic feature, Thaumasia plateau, includes a complex array of small- and large-scale faults and ridges and ancient volcanoes. The large conspicuous Argyre basin, southeast of the Thaumasia plateau, contains a broad expanse of light-colored plains 800 km across. The permanent south polar ice cap is located near the bottom.