The decay curve is a graph showing the relationship of the amount of a substance like Phosphorus-32 with respect to time. Through time, the amount decreases. If the curve was given, you will just have to locate the half of the original amount in the y-axis and connect it to the curve. Then, you move it downwards to the x-axis to determine the time. The time at which the amount of Phosphorus-32 left is reduced to half is called the half-life.
But this have been experimented and there is already an empirical value for this. The half-life of Phosphorus-32 is 14.29 days.