We know that the sum of numbers on opposite faces in dice is always 7.
The given numbers on the faces are 2,3,4 and 6.
So the missing numbers would be 1 and 5.
We know that 2 and a, both the numbers are there on the adjacent faces.
So a can never be 5 because if a = 5, sum of the adjacent faces will become 7.
So a = 1
and b = 5 will be the answer to our question.