suppose the no. is a

a = Dx +8 ...1

a = 3Dy + 21 ...2

So from 1 & 2 we get

D(x-3y) = 13

13 is prime so x-3y has to be 1

let x = 4 and y = 1

We get a = 60

2a = 120

So 120 when divided by 3D = 39

we get remainder as 3

So answer is 3