Question 366861
{{{ N(x)=-.4x^2+10.4x+10}}}
The peak is at {{{x = -b/(2a)}}} when the
equation is in the form  {{{ax^2 + bx + c}}}
{{{a = -.4}}}
{{{b = 10.4}}}
{{{ -b/(2a) = -10.4/-.8}}}
{{{x = 13}}}
Sales will peak 13 days after concert is announced
{{{ N(x)=-.4x^2+10.4x+10}}}
{{{ N(13)=-.4*13^2+10.4*13 + 10}}}
Solve for {{{N(13) }}}