so if my graph is absorbance vs time in minute, then my half life formula should give me time in minute right?
The answer is "yes" but I am not sure what formula you are referring to.
Are you expected to use the absorbance v. time graph to get the half-life? If so,
•Select a point on the graph with a high value for absorbance, A. Read the time in minutes, T1.
•On the absorbance axis, find the point corresponding to half of the value above. Use the graph to find the time for the new absorbance value, T2.
• Half-life, T(half) = T2-T1
If you repeat this process with another pair of points, you should get the same half life. I assume you are dealing with a first order reaction, and the absorbance is proportional to the concentration of a reactant that keeps decreasing.