A tax advisor is simply someone who knows enough about tax law to advise how that law applies to your particular situation. Examples include a certified public accountant (CPA), tax lawyer, or enrolled agent (EA).
In this case you received either stock and/or cash as a result of the merger. Section 302 of the tax code (see https://www.law.cornell.edu/uscode/text/26/302) is about whether or not that redemption of stock in exchange for new stock and/or cash is taxable as a sale or not.
This is complicated and I would suggest that you should seek the guidance of a CPA, EA, or tax lawyer to apply the law to your case.
You can always file an extension and you will have plenty of time to find someone before the extended October 15th filing deadline.