Since acceleration, d2y/dx2 is a constant negative, when dy/dx, velocity, equals zero, it represent the absolute maximum for y...
dy/dx=0 only when 4x=20, x=5
y(5)=7 so the vertex is at the point (5, 7). It has an absolute maximum at that point and decreases without bound on either side of x=5. So the parabola opens downward.