Answer of this question should be
T(n) = T(2*n/3) + 1
because after split there are two possibilities
either T(n) = T(2*n/3) + 1 or T(n) = T(n/3) + 1,
because that element is either present in left side or in right side, not on both sides.
As we need to find the worst case so number of comparison holds this relation 2*n/3 > n/ 3 ,
so we take this case T(n) = T(2*n/3) + 1.