A18 gram sample of a substance that’s used to detect explosives has a k-value of 0.1226

find the substance’s half-life in days
round your answer to the nearest tenth

Given:
Initial mass = 18 g
k-value = 0.1226

The mass at time t (days) is


At half life, m = 18/2 = 9 g.
The time for half life is given by

Therefore
ln(1/2) = -0.1226t

Hence obtain
t = ln(1/2) / -0.1226
  = 5.654 days

5.7 days (nearest tenth)

Where are the following equations?