Original amount (at time zero)

A(0) = 12 mg

Amount at "half-life" time

A(5730) = (1/2) * 12 mg = 6 mg (the definition of half-life)

An exponential decay function is of the form

A(t) = A(0) * e^(-kt), where k is a constant that we need to find

A(5730) = A(0) * e^(-k5730)

6 = 12 * e^(-k(5730)

.5 = e^(-k(5730)

ln(.5) = -5730k

k = 0.00012097

A(t) = 12 * e^(-0.00012097)t