a) When we subtract 2n+3 from 6n+8, we get
... (6n+8) - (2n+3) = 6n - 2n + 8 - 3 = 4n+5
b) Each pile contains half the cards, so has
... (1/2)·(6n+8) = (1/2)·6n + (1/2)·8 = 3n+4
