Question

How do I use the vertex formula to determine the vertex of the graph for `f(x)= -x^2-6x`?

Answer 1

You know the graph of that function is a parabola with vertical axis of symmetry, because it's:

`y=ax^2+bx+c`, with `a=-1, b=-6 and c=0`,

so you can use the vertex formula

`x_v=-b/2a=-(-6)/(-2)=-3`
`y_v=f(x_v)=-(-3)^2-6*(-3)=-9+18=9`

so `v=(-3,9)`