Do I need to arbitrarily assign cardinal numbers to each of these to solve? Thank you!…

Do I need to arbitrarily assign cardinal numbers to each of
these to solve? Thank you!
Consider three finite sets A, B and C. Show that: AUBUC| = |A| + |B| + |C| – |A n B-|An Cl – |Bn Cl + JA n B + C; (a)