The $ y $-coordinate of the vertex is negative, and there are two $ x $-intercepts, so the parabola must be upward facing, which means that $ a $ must be positive. Furthermore, one $ x $-intercept is positive and the other is negative, so the $ y $-intercept $ c $ must be negative. The $ x $-coordinate of the vertex is positive, which is also $ -\frac{b}{2a} $. Since $ a $ is positive, $ b $ is negative. Therefore, the only coefficient that must be positive is $ \boxed{a} $.