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)