。
That's my line, actually.
Literal
That [topic-は] over-here [gen-の] line [copula-です] [emph-よ].
こっちのセリフ '(that is) this side's line' is a stock comeback phrase meaning 'that's what I should be saying to you / no, you're the one who should be X-ing.' Often said when someone accuses the speaker of something the speaker has the better claim to be complaining about. セリフ is the katakana rendering of 台詞 'line / dialogue,' borrowed from theater/drama into everyday speech.