Answer
b is positive, because the x-coordinate of the vertex is positive.
Work Step by Step
The x-coordinate of the vertex is calculated with $x=-\displaystyle \frac{b}{2a}.$
From the graph, we see that the vertex is to the right of the y-axis, so the x-coordinate is positive.
a is negative, so to make x negative, b must be positive.
(When b is positive,
the minus in front of the fraction combines with the negative denominator to give a positive value for x)
