Question 922972

 
The vertex x = -b/2a where these are the coefficients  

you must mean 2y = -4x^2 + 24x +100  not just -4

if so 

 get y by itself by dividing by 2

  y = -2x^2 + 12x + 50

  now  a = -2 and b = 12

 x = - (12/2(-2)) = 3    y = -2* 3^2 + 12*3 +50  
                      y = -2 * 9 + 36 + 50 =  = 68

Vertex is (6, 68)