If the owner of brandX.com has infringed on your trademark with bad faith intent, i.e. they are using it in the same goods/services class, then you may have rightful cause to obtain the domain under ICANN UDRP arbitration or, if you live in the United States, to file suit in federal court under the terms of the ACPA. I highly suggest you review both of the Wikipedia articles I have linked to—they give specific criteria under which you may legally obtain the domain.