2.1->2.4 Poisson/mean and variance of Poisson/adding Poisson distributions

what is the formula for P(X=x)|X~Po(λ)

how could you derive this?

for which 3 conditions would a Poisson distribution be a good model?

what is the mean of a Poisson distribution

E(X)=λ

what is the variance of a Poisson distribution

Var(X)=λ