Solve for X $\mathrm { A } = \mathrm { Ce } \mathrm { e } ^ { - 0.1813 \mathrm { t } }$

Solve for x.
-The radioactive decay of radon 222 can be described by the equation $\mathrm { A } = \mathrm { Ce } \mathrm { e } ^ { - 0.1813 \mathrm { t } }$ , where $\mathrm { C }$ is the original amount of radon and $\mathrm { A }$ is the amount of radon after $\mathrm { t }$ days. If $2.5$ milligrams are in a laboratory container today, how much was there is the container 11 days ago? Round to the nearest hundredth.

A) $2.84 \mathrm { mg }$
B) $0.34 \mathrm { mg }$
C) $45.68 \mathrm { mg }$
D) $18.37 \mathrm { mg }$