Answer
$(-\infty, 0] \cup [4, +\infty)$
Work Step by Step
Notice that the value of $x^2-4x$ (which is represented by the parabola) is greater than or equal to 0 in the following intervals (refer to the graph in the attached image for reference):
$(-\infty, 0]$
$[4, +\infty)$
Therefore, the solution to the given inequality is:
$(-\infty, 0] \cup [4, +\infty)$
