Answer
$a<0$
Work Step by Step
The equation of a parabola is $$y=ax^2+bx+c=a(x-h)^2+k.$$ A parabola that opens downward has the value of $a$ to be negative, while a parabola that opens upward has the value of $a$ to be positive.
In our case the parabola opens downward, therefore $a<0$.
