Hi,
Note:the vertex form of a parabola,where(h,k) is the vertex
N(x)= -0.4x^2+12x+11
N(x)=-0.4[x^2 -30x]+11
N(x)=-0.4[(x-15)^2 -225]+11
N(x)=-0.4[(x-15)^2 -225]+11
N(x)=-0.4(x-15)^2 +101 V(15,101) a = -.4 a<0, Parabola opens downward
V(15,101)is the Maximum Point. ticket sales peak on the 15th day with 101 sold