Deciding if a relation is a function

Hi everybody I'am going over some of the problems in my textbook preparing for my exam on monday and I have some doubts with the following problems:

"Decide whether each of the following relations is or is not a function and give the

type of those relations that are functions. Justify your answers.

a) R = {(a, b): a, b real and a + 3b2 = 1 }

b) R = {(a, b): a, b integers and floor(a + b) < ceil(b)}

c) R = { ((a, b), c) : a, b and c integers and a + b + c = 0}

d) R = {((a, b), c): a, b and c subset of a nonempty set A, and a U b = c} "

a and b i think I understand although I not 100% sure if I'm right but I think neither of them are functions.

c and d are giving me some trouble especially d, I think c is an algorithmic function b/c there is a unique solution for c which is -1*(a+b), for d i really have no clue any help will be appreciated thanks.