(d)if X1+X2 = 5 then one of them must be 2, ie:

X1=3, X2=2

OR

X1=2,X2=3

Show that the probability of X1=2 is 0, and likewise for X2.

(e)

X1, X2 can be 0,1,3. There are only 9 possibilities for the combination. Calculate them all and hence the distribution of X1+X2.

(f)

use the distribution you calculated in (e).