する verb
はってんする
年寄りの風邪は肺炎に発展しやすいんだよ。
When old people catch a cold, it can easily develop into pneumonia.
Let's depart from this place and expand it out. To do that we'll have to develop it.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The はつ gets shortened to はっ, though.