I've sent this to 3 different tutors and they all got this answer wrong. Please help

The concentration C in milligrams per milliliter (mg/ml) of a certain drug in a person's blood-stream t hours after a pill is swallowed is modeled by C(t) = 9+ - e -0.081 Estimate the change 1+ +3 in concentration when t changes from 10 to 20 minutes. The change in concentration is about | mg/ml. (Type an integer or decimal rounded to the nearest thousandth as needed.)