What does s^2t-st^2 equal given that s and t are positive integ

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Your original question talked about s and t being greater than and then you mention they are greater than 1 as well. It only needs to mention that both s,t are >1.

Also, prime can ONLY be integers and as such mentioning both in statement 1 is redundant. GMAT language is a lot more subtle and to the point.

As for this question,

\(s^2t-st^2\) = ? with s and t \(\in\) integers > 1.

Per statement 1, t= prime number . Too many options for t and no clue about s, making this statement not sufficient.

Per statement 2, st=15 ---> as both s and t>1 and 15 has 1,3,5,15 as the factors, only (3,5) or (5,3) will work as (s,t) pair. But it is also given that s>t and thus (3,5) is eliminated , giving us the only possible values of s,t as 5,3 and hence you will get a unique value of \(s^2t-st^2\) (no need to solve this final unique value).
This statement is sufficient.

B is the correct answer.