Algebra

Several ways to do this:

  find the derivative and set = 0

    0 = -4x + 18       shows x = 4 1/2      use this value in the original equation to find the 'y' coordinate

 

 

Arrange  the parabolic equation into vertex form

    -2 ( x^2 - 9x)    - 15

    -2 ( x -4.5)^2  +25.5                shows  h,k   =   4.5, 25.5

 

 

Third way

  max will occur at   - b /2a   = -18/(2*-2) = 4 1/2     use this value in the original equation to calculate 'y'