To Promise
する verb, transitive verb
約束 is a “promise.” So when you add する to it you’re making it a verb, meaning to promise. Now promise you’ll remember!
やくそくする
The reading is the same as 約束 with する added. Make sure you know 約束 and you'll know this word as well.
「約束する？」「うん、約束するよ。」
"Promise?" "Yes. I promise."
ゼッタイに誰にも言わないって、約束してくれる？
Can you promise that you won't tell this to anybody else?
大好きな人とデートの約束したんだけど、インフルになって行けそうもなくて泣いてる。
I made plans for a date with someone I really like, but I got the flu and now I'm crying because it seems like I won't be able to go.