Refer to the following:
During the course of treatment of an illness, the concentration of a drug in the bloodstream in micrograms per microliter fluctuates during the dosing period of 8 hours according to the model
$$
C(t)=15.4-4.7 \sin \left(\frac{\pi}{4} t+\frac{\pi}{2}\right), \quad 0 \leq t \leq 8
$$
Note: This model does not apply to the first dose of the medication.
Find the concentration of the drug in the bloodstream 6 hours after taking a dose of the drug.