Question 472475
In this case vertex is the high point of the parabola (ticket sales peak) because coefficient a= -0.4 is a negative number and the parabola opens downward
To answer this question you have to find coordinates of the vertex of parabola N(x)= -0.4x^2+12x+11 where x is number of days and y = N(x) is number of tickets sold

The general form of quadratic equation is y= ax^2+bx+c

x-coordinate of vertex {{{x= -b/(2a)}}}
y-coordinate of vertex {{{y=-d/(4a)}}} where d= b^2-4*a*c

Substitute for a, b, and c  and you should get answer that on 15th day x=15 they will sell the most tickets y=101