Answer
approximately $27.44$ milligrams
Work Step by Step
So, we have a function of $D(t)=50e^{-0.2t}$
Simply inputing $t$ hours in the expression will give us the number of milligrams remaining in the patient's bloodstream after that specific $t$ hours.
For $t=3$
$D(3)=50e^{-0.2\times3}=50e^{-0.6}\approx27.44 milligrams$
