title: how to define the probability distribution
body: <p>I have small question and I will be very happy if you can give me a solution or any idea for solution of probability distribution of the following idea: 
  I have a random variable x which follows exponntial  distribution with parameter lambda1,I have one more variable y which follows exponential  distribution with parameter lambda2. z is a discrete value, how can I define the probability distribution of  k in the following formula ? </p>
  
  <p>k=z-x-y </p>
  
  <p>Thank you so much </p>