Question 919758
the vertex form of a Parabola opening up(a>0) or down(a<0), 
{{{y=a(x-h)^2 +k}}}. V(h, k)
 Completing the Square to Obtain the Vertex Form:
y = ax^2 + bx + c = 0
p=-h^2+60h-400 = 0 {{{b/(-2a) = (60)/(1) = 30}}} 30 the x-value for the Vertex
p = -(h - 30)^2  + 900 - 400= 0  {{{-a(b/(-2a))^2 }}}= 900
p = -(h - 30)^2  + 500
V(30, 500)
h = $30, Profit $500

{{{drawing(300,300,   -100, 100, -700,700, grid(1), blue(line(2.5,6,2.5,-6)) ,
circle(30,500,2),
graph( 300, 300, -100, 100, -700, 700,0, -x^2+60x-400 ) )}}}