<span>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. </span>
<span>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. </span>
<span>Let a and b be any two values on [-8, ∞), such that a < b. </span>
<span>-8 ≤ a < b </span>
<span>Then f(a) > f(b). Therefore, function f is decreasing on interval [-8, ∞).</span>
