What is the probability that an f2 seed chosen at random will be yellow?

Well, all of the possible results or events are expressed by the description of generation F2:? = {YY (yellow), Yy (yellow; dominant trait), Yy (yellow; dominant trait), yy (not yellow)}
n(?) = 4

You can do this intuitively: three out of four seeds are of yellow specimens, so there's a 3/4 or 75% probability that yellow will be chosen.

Or you can do it by the theoretical definition of probability, using a formula:
p(A) = n(A) / n(?), where p(A) stands for "probability of happening the event A", n(A) stands for "number of elements or of individual events that satisfy the definition of A", and n(?) stands for "total number of possibilities; number of elements in the sample".
Being event A "choosing a seed of any yellow specimen", n(?) = 4 (4 genetic possibilities) and n(A) = 3 (3 out of all of the elements in ? satisfy the event, or, if being chosen, would mean that the event has happened, as 3 out of 4 seeds are yellow, according to genetics):
p(A) = 3/4 or 75%

Both ways, you've got your answer: the probability that an F2 seed chosen at random will be yellow is of 75% or 3/4.

Was my answer clear? I hope you could get it. And I hope I've helped!