Hpw to find exponential decay equation of radioaxtive subsrance

I believe you are referring to the half-life formula, which is used to calculate the half-life (when the substance drops to half of its original amount) of radioactive substances.  The equation is as follows: 

A is the remaining amount
P is the initial amount
t is the time that has passed
h is the half-life of the substance

When plugging values into the equation, make sure that t and h are both measured in the same unit (e.g. t days and h days), as well as A and P.