f(x) = -(x + 8)² - 1
The function (not its graph) decreases on interval [-8, ∞). It is a quadratic function in vertex form. That form makes it easy to pick out the one extremum, where x equals -8. The leading coefficient is negative, so the extremum must be a maximum. The function decreases as x increases from there.
Notice that I include the value -8 in the interval. The function does not have instantaneous decrease at that value, but that is not what it means for a function to be decreasing over an interval.
Let a and b be any two values on [-8, ∞), such that a < b.
-8 ≤ a < b
Then f(a) > f(b). Therefore, function f is decreasing on interval [-8, ∞).
