#### Answer

a. 1.5 mg/L
b. $C(3)= 1.5\ mg/L$
c. $y=0$. See explanations.

#### Work Step by Step

a. Based on the given graph, we can get a concentration value about 1.5 mg/L at $t=3$ hours.
b. Using the equation, we have
$C(3)=\frac{5(3)}{3^2+1}=1.5\ mg/L$
c. As $t\to\infty$, we have $C\to0$; thus the horizontal asymptote can be found as $y=0$. This means that the drug concentration will approach to zero as time increases.