One way to locate the vertex is to obtain the coefficients a, b and c from the quadratic and then calculate
-(-16)
x = -b / (2a). Here, x = ---------- = -2
2(-4)
Next, evaluate y at x = -2: y = -4(-2)^2 - 16(-2) - 12 = -16 + 32 - 12
y = 4
the vertex is at (-2,4).