Respon why don't you join us for lunch

i already had my lunch.. so i'm full now.
kayak gitu ?

Gak ngerti soalnyaa T^T jadi mungkin itu kita ngasih respon gitu? T^T
A: Why dont you join us for lunch?
B: I must finish my Math homework. I dont want my teacher kill me T^T
A: Okay then. But dont forget to lunch
B: 'Kay. I'll lunch later,