I recently purchased a house, and found that the cable drop from power line. not sure is that cable belongs to Xfinity.

Thank you for reaching out to us @frankccl0725! Our team can certainly check if this line is ours by opening a ticket for our local team to inspect the line. Could you please send our team a direct message with your full name, full address, phone number, and email address? To send a "Direct Message" to Xfinity Support:

Click "Sign In" if necessary
Click the "Direct Messaging" icon or https://forums.xfinity.com/direct-messaging
Click the "New message" (pencil and paper) icon
The "To:" line prompts you to "Type the name of a person". Instead, type "Xfinity Support" there
 - As you are typing a drop-down list appears. Select "Xfinity Support" from that list
 - An "Xfinity Support" graphic replaces the "To:" line
Type your message in the text area near the bottom of the window
Press Enter to send it.