The polynomial

$\displaystyle {y}=-{0.77}{x}^{{4}}+{3}{x}^{{3}}+{25.4}$

describes the billions of flu virus particles in a person’s body $\displaystyle {x}$ days after being infected. Find the number of virus particles, in billions, after 4 days.

There are billion virus particles in a person’s body 4 days after being infected.