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Answer :

By definition of f(x),

f(k) = 1 - 2k + k = 1 - k

Then

f(f(k)) = f(1 - k) = 1 - 2 (1 - k) + k = 3k - 1 = 13

Solve for k :

3k - 1 = 13

3k = 14

k = 14/3

Find f(-1) :

f(-1) = 1 - 2 (-1) + 14/3 = 1 + 2 + 14/3 = 23/3

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