The group $S_3$ is defined as $\langle a,b \mid a^2 = b^3 = 1, ba = ab^2\rangle$, where $a = (12), b = (123)$.
The elements $ba$ and $ab ^ 2$ are both equal $(23)$. Since there is a homomorphism from $G$ to $G/H = S_3$, the elements $ba$ and $ab ^ 2$ are in the same coset. Therefore, the Schreier representative $ba$ is equal to $ab ^ 2$.