What is / how do you calculate the vertex for a parabola with this equation: y=-1x^2 13x?

Y=x^2+13x

complete the square to get it into form y=a (x-h) ^2+k whre (h, k) is the vertex

y=x^2+13x

y = (x^2+13x)

13/2=6.5, 6.5^2=42.25

y = (x^2+13x+42.25-42.25)

y = ((x+6.5) ^2-42.25)

y = (x - (-6.5)) ^2 + (-42.25)

the vertex is (-6.5,-42.25)