Mexicali, the capital city of the Mexican state of Baja California, is located 300 kilometers southeast from Los Angeles. No capital city is located 300 miles southeast of Los Angeles. Phoenix, Arizona is 360 miles east of Los Angeles.
"The capital city southeast of Los Angeles" is not a well-defined term, so it's impossible to answer the question as asked. Mexico City is "southeast of Los Angles", if we're not too picky about the definition of "southeast". So are most of the capital cities of Central and South America. (So is Miami FL, for that matter.)
Asuncion is the capital and largest city of Paraguay. It is located 660 miles miles southeast of Sucre, Bolivia.
Phoenix, Arizona
If you start in Los Angeles and travel about 300 miles southeast, you would reach Phoenix, Arizona, which is the capital city of Arizona. From there, traveling north for 400 miles would take you into northern Arizona, and then heading west for another 200 miles would bring you into California. Therefore, you would end up in California.
Sanna, Yemen lies 495 miles southeast of Mecca.
The capital that is 325 miles north of Istanbul is Sofia, Bulgaria. Meanwhile, a city located 230 miles southeast of Istanbul is Ankara, which is actually the capital of Turkey. Therefore, Sofia is the capital north of Istanbul, while Ankara is the capital southeast of it.
Riyadh is 270 miles southeast of Bahrain.
Phoenix, Arizona is about 287 miles southeast of Las Vegas, Nevada.
Belarus
Brussels, the capital of Belgium, is 200 miles east southeast from London, England.
Capital City Airport is located near Harrisburg. More precisely, Capital City Airport is located 3 miles southeast of Harrisburg, Pennsylvania which is the capital of Pennsylvania.
Sanna, Yemen lies 495 miles southeast of Mecca.