dpoist gives you the probability mass at a single count value. You can pass it several count values and it will give you the probability mass at each one. ppois gives you the sum of all of the probability masses from zero up to given count value. Here are three results that illustrate that:
> dpois(0:12, lambda = 13)
[1] 2.260329e-06 2.938428e-05 1.909978e-04 8.276573e-04 2.689886e-03 6.993704e-03 1.515303e-02
[8] 2.814133e-02 4.572967e-02 6.605396e-02 8.587015e-02 1.014829e-01 1.099398e-01
> sum(dpois(0:12, lambda = 13))
[1] 0.4631047
> ppois(12, lambda = 13)
[1] 0.4631047
Since your question seems to be from homework, I want to avoid simply giving you the answers.
You should know what is the sum of all of the probability masses from zero to infinity.
You can calculate the probability of all results up to any number
Knowing those two things, you can get the probability of getting a result greater than a number.