Hi
Using the vertex form of a parabola,  where(h,k) is the vertex
 where(h,k) is the vertex
f(x) = -x^2 + 6x   |Completing Square to find the Vertex
y = -[(x-3)^2 - 9]
y = -(x-3)^2 + 9      |represents parabola opening downward, a<0 
vertex Pt(3,9) f(3) = 9 is a maximum point
Domain:  and Range: {y|y ≤ 9}
 and Range: {y|y ≤ 9}