A random sample $X_1, X_2, . . . , X_5$ is taken from a Poisson distribution with parameter λ for some λ > 0. Find the joint probability mass function in as simplified a form as possible for ($X_1, X_2, X_3, X_4, X_5$) if the observed values of the random sample are (2, 4, 0, 3, 5).
How should I approach this problem? I don't know where to start!!