HOW TO FIND HALF-LIFE
You can find the half-life of a drug from the graph showing how a drug's concentration changes over time where the graph is showing first order elimination (ie where the semilogarithmic graph is a straight line).
The following page goes through this method using a semilogarithmic graph plotted for an example drug given as a single intravenous bolus.
Note: You can use equations to calculate half-life. However, this package concentrates on finding half-life directly from a semilogaritihmic graph.