Okay I'm gonna help you out. Let's make things easier.
Let A = {a,b}
and B = {x,y}
Assuming that a ≠ b ≠ x ≠ y
If I see their cardinality, (note that there only 2 elements in either of the sets which is finite) then,
|A| = 2......(1)
|B| = 2........(2)
From 1. and 2,
|A| = |B|
By the definition of addition,
|A| + |B| = 2 + 2 = 4
Hope you understood the underlying concept, i.e., if you take 2 2s and put them together they'll always result in 4.