The half-life is defined to be the time it takes for any specified property, in this case, mass, to be half of its original value. Thus, given the equation:
N = No*e^-kt, with N = 9, No = 18, and k = 0.1226, the value of t, in days, can be determined. Solving the equation, the half-life of the substance, or the time it takes for the sample to be 9 grams is 5.7 days.