"The number of subsets of set A is twice the number of sub sets of B."
x(A) = 2x(B)
"Set B is larger than set C."
So we know that x(B) > x(C). And I believe you mis-typed the last part of the equation where you said "Number of subsets of C is 15 more than number of subsets of C." The first C would have to be a B for this question to make sense. But nonetheless, this statement tells us that:
x(B) = x(C) + 15
And we are asked to find x(A).
x(A) = 2x(B) = 2[x(C) + 15] = 2x(C) + 30
This is the closest one can get to the answer with the given information because there needs to be a value for x(C), or at least a way to figure it out.