Complex sentence: On 6 December 1829, the House of Representatives of Buenos Aires elected Rosas governor and granted him "faculdades extraordinarias" (extraordinary powers)--in other words, "unbridled dictatorial powers".
Split into simpler sentences: On 6 December 1829 the House of Representatives of Buenos Aires elected Rosas governor and granted him "faculdades extraordinarias" (extraordinary faculties). In other words, "unbridled dictatorial powers".