Conditional Expectation

SOLVED

Consider a Poisson process with parameter $\lambda$ . Let $X$ be the number of events in $(0,3]$ and $Y$
the number of events in $(2,4]$ .
(a) Find the mean and variance of $Y-X$ .
(b) Find $E(Y | X)$ . Verify that $E[E(Y | X)] = E(Y)$ .

I've done (a), and got the answers $-\lambda$ and $3\lambda$ for mean and variance respectively.

I can't figure out (b) - I know the formula for calculating conditional expectation, but can't seem to figure out how to find $f_{Y|X} (y|x)$ (more specifically, I'm not able to calculate $P(X=x, Y=y)$ ) since the two variables are not independent.

EDIT: I've made some progress in calculating $P(X=x, Y=y)$ :

It's easy if $x,y$ were actually known integers, but since they're not I figured that we could let the number of events in $(2,3]$ be denoted by $a$ . Hence we get:

$P(X=x, Y=y) = \sum_{a=0}^{\min(x,y)} (\frac{\lambda^{x-a}}{(x-a)!}e^{-2\lambda})(\frac{\lambda^a}{a!}e^{-\lambda})(\frac{\lambda^{y-a}}{(y-a)!}e^{-\lambda})$

..and I'm stuck.

SOLVED