Short answers, pulled from the story.
Marcus Aurelius Severus Alexander became Roman emperor on the 13th of March 222 at age thirteen. He was the grandson of Julia Maesa who orchestrated a political coup to place him on the throne after his cousin Elagabalus was murdered.
Severus Alexander revalued coinage in 229 raising silver purity to 45% and weight to 1.46 grams before further increasing it to 50.5% and 1.50 grams the following year. He lightened taxes and instituted loan offices for moderate interest rates while restoring the Baths of Nero around 227 or 229.
Ardashir invaded Roman provinces overrunning Mesopotamia and penetrating possibly as far as Syria and Cappadocia in 231 AD. Alexander organized a three-fold invasion in 233 making Antioch his base but suffered humiliating setbacks and losses due to disease and inclement cold in Armenia.
Legionaries mutinied against Severus Alexander in early months of 235 at Moguntiacum because they considered his decision to buy off Germanic tribes dishonorable. Soldiers compared him to cousin Elagabalus and feared he was unfit to be Emperor due to these actions leading to his death along with mother Julia Mamaea.
Death marked end of Severan dynasty and first emperor overthrown by military discontent on wide scale signaling beginning of chaotic period known as Crisis of Third Century. Emperors increasingly expected display general competence in military affairs following his death while Gaius Julius Verus Maximinus emerged as next best option from Thrace.