Find the vertex for f(x)=-5x^2-6x=3

Answer 1

It appears that the problem should read:
f(x)=-5x^2-6x+3

complete the square:

-5x^2 - 6x + 3
-5(x^2 + 6/5 x) + 3
-5(x^2 + 6/5 x + 9/25) + 3 + 5*9/25
f(x) = -5(x+3/5)^2 + 24/5

vertex is at (-3/5,24/5)