What is the vertex of y=-6(2.5-x)(x-5.5)?

Hello : 

 y=-6(2.5-x)(x-5.5) = -6(2.5x -13.75 -x² +5.5x)
y = -6(-x²+8x -13.75)
 y = 6x²-48x+82.5
note : 
if f(x) = ax²+bx +c   the vertex is the point : ( -b/2a ; f(-b/2a))
a=6  b=-48 c = 82.5 calculate
-b/2a = -(-48)/2(6)= 4
f(4) =6(4)²-48(4)+82.5 =96 - 192 +82.5 = -13.5

)because it's going in order of the math sign