What is the minimum number to be subtracted from 6321, which makes it completely divisible by 14?

A number is divisible by 14 only when it is divisible by both the factor of 14 i.e 7 and 2 

we know a number is divisible by 2 when the last digit of the number is even

we also know that a number is divisible by 7 when the double of last two digits of the number, when subtracted by the remaining number gives 0 or different multiple of 7

so if we subtract 7 from the number we get an even number and a number divisible by 7 hence making the number divisible by 14