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1) The maximum possible value for f(2) under the question is 2 + 6*2 = 2 + 12 = 14.
2) can f(5) be negative ?
ANSWER. YES. Take, for example, f(x) = 2 - x.
Then f(0) = 2 and f(5) = 2 - 5 = -3,
and f(x) satisfies all imposed conditions.
can f(5) be equal to 0 ?
ANSWER. YES. Take, for example, f(x) = 2 - 0.4x.
I answered all the questions.
The function f(x) is assumed to be continue and differentiable in the given interval (in order for the question makes sense).