If the number of complaints in 1 week is poisson with mean 2, then the number of complaints in 3 weeks is poisson with mean 6.

(NB: this is a special property of the poisson distribution, it is not true for all other distributions).

Now, the paramter of a poisson distribution is equal to the mean; so the number of claims in 3 weeks is

You can find the chance that x=3 using the PF of a poisson distribution

You can use similar arguments for part b.

Note: You may have been taught a different formulation of the poisson distribution where the parameter is (1/mean). In this case, the answer is the same but the method needs adjusting somewhat.