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a)

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First think of length(y). This is $5$ because y is the sequence of numbers between $2$ and $6$, inclusive, i.e. {join(list(2..6),' ')}.

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Then y[3:length(y)] means extract the third to fifth element of y, i.e. 4 5 6.

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b)

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Similarly, extract the second to third elements of x because length(x) = 3.

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c)

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The rev() function reverses the order of the given vector. 

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d)

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which(y==4) gives the index of the first occurrence of $4$ in y. The third element of y is $4$ therefore $b=3$.

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> y = seq(2,6)
> x = y[3:length(y)]
> x
> z = x[2:length(x)]
> a = rev(z)
> b = which(y==4)
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