Find vertex of f(x)=(x+3)(x-5)

The x coordinate of the vertex will be the average of the two zeros, here -3 and 5, so x=(-3+5)/2 = 1, f(1)=(1+3)(1-5) = -16.

(1, -16)

Let's do it some other ways.  How about completing the square to turn f in to vertex form?

f(x) = (x+3)(x-5) = x² - 2x - 15 = (x² - 2x + 1) - 1 - 15 = (x-1)² - 16

and now we can read off (1, -16) as the vertex.

The other method is the vertex is x= - b/2a =  - (-2)/2(1) = 1.

Three methods, same answer. Good.