Sorry, that is not right ! From where do you have b+x as well as x-a ? What are the steps that you used to obtain the above step ?
You have
Step 1 :- What do you have after making the necessary substitutions as explained by my hint? Remember that you want it as powers of x-a.
Step 2 :- How do you expand ? In this problem,what do you have in place of y ?
Later we can discuss the validity of convergence.