Because there is a remainder.....subtract the remainder 12 from each given number first.......then find HCF of the numbers left after subtraction......and to find the HCF....find all the prime factors of each....then multiply together all the factors that are common......This gives the highest common factor......
Subtract 12 from each number gives 1344, 1856, 2752
Find the prime factors of these numbers, then multiply together all the factors that are common to each to find the Highest Common factor HCF ........ This works out to be .... 64
Check.....
1356/64 = 21 rem 12
1856/64 = 29 rem 12
2764/64 = 43 rem 12
So..... 64 is the biggest number that divides into each to leave a remainder of 12.