Answer
The solution set is $(-\infty,-4]$
Work Step by Step
The idea is to isolate x on one side.
Adding, subtracting or multiplying/dividing with a positive number preserves order (the inequality symbol remains).
Multiplying/dividing with a negative number inverts the order (the inequality symbol changes).
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$-9x \geq 36\qquad $ ... $/\div(-9)$, (negative!)
$ x \leq -4$
The solution set is $(-\infty,-4]$
