Doubt from permutations and combinations

I was able to find b and c part but don't know how to approach part a.

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the situation can be interpreted in a different way as no. of onto function from set A to set B where n(A)=6(balls) and n(B)=3(boxes)
as every balls have to put in boxes and each box should have at least 1 ball
so you have to apply the formula of no of onto functions @Vanshvardhan_2020

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