a)I think you're wrong. two sets of 10 elements may represent the same combination of colors.
b)it's easy, you have 3 possibilities for each element in the sequence, so 3^5.
c)each color combinations of 10 beans that have exactly two yellow beans can be represented by a color combination of 8 beans that have only red and green, so you count how many combinations of 8 red/green beans you can have.
d)think of the complement of a set. you use the result of question a) and remove the number of combinations of 10 beans that have 1 or 0 yellow beans.
ps : maybe i'm wrong, i'm not sure what they mean when they say a color combination, are two sets of different beans but with same colors considered as two different combinations ? (i supposed no)