I am having difficulty with the following problem. I tried conditioning on T_n but I am unsure how to proceed with that conditional expectation. Thanks for the help!

I am having difficulty with the following problem. I tried conditioning on T_n but I am unsure how to proceed with that conditional expectation. Thanks for the help!

$\Pr(N(t)=n\text{ and }T_{N(t)+1} > t+s)$ is the probability that the number of renewals before time $t$ is exactly $n$ and the time until the next renewal after that is more than $s$. This is $\Pr(N(t)=n) \cdot \Pr(T_{N(t)+1} > t+s)$, because the two events are independent because they are about non-overlapping time intervals in a Poisson process. The waiting time until the next renewal is exponentially distributed with mean $1/\lambda$. So the probability that it exceeds $s$ is $e^{-\lambda s}$. Hence the probability you're looking for is $$ \frac{(\lambda t)^n e^{-\lambda t}}{n!}\cdot e^{-\lambda s} = \frac{(\lambda t)^n e^{-\lambda (t+s)}}{n!}. $$