X=number of accidents.

where x=0,1,2,3....

Yes, (a) is P(X<2) = P(X=0)+P(X=1)

(b) P(X>4)=1-[P(X=0)+...+P(X=4)]

In (c) I assume we are talking about 3 indep sites all Poisson with mean .5.

The sum of indep Poisson's is a Poisson.

where y=0,1,2,3....

and you want