User blog:Mgf60/Vol.1 Ch.58 Naver Endnote translation explanation

Original Korean: 너는 아 7ㅓ다

너는 - you (subject)

아 - oh/ah (surprise)

"7ㅓ다" have no meaning in itself, but the seven is used to replace one of the letters in "거", which turns "7ㅓ다" into "거다".

거다 - will

This turns the sentence into "Oh, you will"

The seven might have been used to symbolize luck, as it is a lucky number in Korea. However, it might also just be a stylistic choice.