There will be
billion virus particles in a person’s body 3 days after being infected.
Explanation:
Step 1:

The number of flu particles in a person's body in the billions is represented by y and x represents the number of days after which they have been infected.
So if we need to calculate the number of flu particles in a person's body at any instant of time we just substitute the value of x in y.
Step 2:
To calculate the number of flu virus particles in a person's body after 3 days, we must substitute
in y.

So 3 days after being infected there will be
billion flu virus particles in that particular person's body.