Question

a 56 kg miniature horse weighs 268 fewer kilograms than shetland pony

Answer 1

If the mini horse is 56kg and it's 268kg less than a Shetland pony then a Shetland pony would weigh 56+268=324kg

Answer 2

let
x--------> Shetland pony's weigh
y--------> Miniature horse's weigh

we know that
$y=56 kg$
$x=268+y \\ x=268+56 \\ x=324 kg$

the answer is
Shetland pony's weigh is $324 kg$