How to know if a virus alert is legit?

Hi! I was doing homework of my science class and suddenly appears a virus alert that I have 4 virus and I need to take action the company is McAfee and it says that my membership expired and I need to pay or something like that. I couldn’t take a screenshot because the advice disappeared but it take me to this page. I want to know if this is spam or I really need to care about

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