What is the value of the x-coordinate of the vertex of the function shown below?

f (x) = (x+4) (x-2)

... ?

f (x) = (x+4) (x-2) = x² + 2x - 8

to find the vertex of a quadratic function, this is the main formula:

for f (x) = ax² + bx + c, and if M (ao, bo) the vertex, so

ao = - b/2a, and bo = 4ac - b² / 4a

so the vertex of f (x) = x² + 2x - 8 is M (-1, - 9)