Consider u = {x|x is a positive integer greater than 1}.

which is an empty set?

{x|x ∈ u and 1/2x is prime}
{x|x ∈ u and 2x is prime}
{x|x ∈ u and 1/2x can be written as a fraction}
{x|x ∈ u and 2x can be written as a fraction}

Hence, the set that is empty is:

{x|x ∈ U and 2x is prime}

We are given:

U = {x|x is a positive integer greater than 1}

i.e. U={2,3,4,5,6,7,}

The empty set among the following is:

{x|x ∈ U and 2x is prime}

As x ∈ U.

This means that x has to be an positive integer which is greater than 1.

and 2x will be an even integer i.e. {4,6,8,}.

Hence,

the term 2x can't be a prime as each of the number has more than 2 factors other than 1 and number itself.

go and ask your mom to you