I don't understand how to do this problem if anyone could help that would be great

Determine which of the following define equivalence relations in R^2. If the relation fails to be an equiv- alence relation, identify at least one property that does not hold and give an example to demonstrate. If the relation is indeed an equivalence relation, give a geometrical interpretation of the quotient set.

(a.) (a,b) ~(c,d) if and only if ab = cd

(b.) (a,b)~(c,d)if and only if 2a + 3b = 2c + 3d

(c.) (a,b) ~(c,d) if and only if a + b^2 = d+c^2