Step 1: Understand the properties of groups.
In a group, the equation \( x * a = b \) has a unique solution, which is given by \( x = b * a^{-1} \).
Step 2: Conclusion.
Thus, the unique solution is \( b * a^{-1} \).
Final Answer:
\[
\boxed{b * a^{-1}}
\]