The diagram shows part of the graph of the function: y=ax^2+bx+c.

(seeing as I cannot draw the diagram here, it is basically an upside down parabola, with the vertex on the x-axis, and the whole parabola is in the fourth quadrant)

State whether each of the following expressions is positive, negative or zero.

a) a negative

b) c negative

c) b^2-4ac zero

d) b ?

I cannot figure out how to find whether constant b is positive, negative or zero. Can someone help me?