The function

$f(t)=-t(t-21)(t+1)$

has been used to model the measles virus concentration in an infected individual. The area under the graph of $f$ represents the total amount of infection. At $t=14$ days this total amount of infection reaches the threshold beyond which syntiptoms are displayed. Use the Evaluation Theorem to calculate this threshold value. $($Round your answer to the nearest whole number.$)$

$q,($number of cells per mL $)-$ days