Because Meg gets an additional 6 ponies, she adds them to her existing 26 ponies. This makes her have 32 ponies.
(CHECK: 26+6=32, 32-6=26)
Since she displays half of her 32, that means 16 of her ponies are on display.
(CHECK: 32/2=16, 16+16=32)
Hopefully this helps!! :)