Question

what is the charge of carbide ion

Answer 1

Its -4
When it becomes carbide, it gains 4 electrons.
Like in Al4C3

Answer 2

the carbide ion consists of two carbons with a triple bond in between them causing the whole ion to have a total 2 unpaired electrons to pair them up two extra electron come over giving the ion 2- charge. And as Ca atom is having a double positive charge(2+).