Answer:
(-∞, -1]∪{5}
Step-by-step explanation:
We know that:
f(x) ≤ 0 is true if the graph of the function is intersecting the x-axis or below the x-axis.
So we can just look at the graph, we can see that in the interval:
(-∞, -1] (the value x = -1 is included, that is why we use the symbol ] instead of the parenthesis)
the graph is below or intersecting the x-axis
and this also happens at x = 5 or just denoted as {5}
Then the correct notation is:
(-∞, -1]∪{5}
