Answer
Positive, because the graph of f is a parabola that should open upwards.
Work Step by Step
The midpoints of the tops of the bars form a concave curve,
roughly resembling a parabola.
The graph of f(t) should be as close as possible to these midpoints.
The parabola opens UP so we want the leading coefficient of f(t) to be positive.
Positive, because the graph of f is a parabola that should open upwards.
