Complex sentence: Momota's mother entered her daughter for an open audition for Stardust Promotion; and when still in the fifth grade of elementary school, Kanako Momota won the audition and was subsequently signed to the agency.
Split into simpler sentences: Momota's mother entered her daughter for an open audition for Stardust Promotion. As a fifth-grader in elementary school, Kanako Momota won the audition and was subsequently signed to the agency.