I keep getting a notification saying that my MacBook Air has been hacked

Please see the very informative user tip below. 

Stop unwanted Notifications - Apple Community

..cemi2525..

This may be adware propagated by unofficial scam phishing attempt,

to use fear to allow access your personal data and private information.

(Even if not that deep, take advantage of opportunity to learn.)

•If you think your Apple Account has been compromised - Apple Support

•Security and your Apple Account - Apple Support

•Change your Apple Account password - Apple Support (all methods)

•Manage and use your Apple Account - Apple Support

•Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Don't trust those fake notices. Mac is quite safe!