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Answer:
f(x) = {x ≤ 0: -x, 0 < x ≤ 4: 4 -x}
Explanation:
The graph is of the line y = -x for x ≤ 0 and of the line y = -x +4 for the interval (0, 4].
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Additional comment
There's not a really good way to specify piecewise functions in plain text. The method used in the Answer section matches that used by the Desmos online graphing calculator. It is of the form {condition: value, condition: value, ...}.