Finding the vertex help thanks!

Complete the square on y = -2x^2 + 8x -15.

Then y = -2(x - 4)^2 + 17, so the vertex is (4,17).

y  =  -2x² + 8x - 15

Move the '-15' term to the other side by adding 15 to both sides:

     y + 15  =  -2x² + 8x

Factor out the '-2' term:

     y + 15  =  -2(x² - 4x)

Complete the square of x² - 4x     --->     x² - 4x + 4

Notice that the '4' will be on the inside of the factor, so really, it is a '-8':

     y = 15 - 8  =  -2(x² - 4x + 4)

Finishing:

     y + 7  =  -2(x - 2)²

The vertex occurs at (2, -7)

We  have  the form   y = ax^2  + bx  + c

The  x coordinate of the  vertex  occurs  at  - b / (2a)  =  -8/ ( 2 *-2)  =  -8 / -4  = 2

To find the y coordinate...put x  back into  the function

y= -2(2)^2  + 8(2)  -15  =    -8  + 16  - 15 =  -7