Hi, i'm not Al but i thought i'd try to help explain. You're right that any equation can be divided, multiplied or whatever, by any number that you want as long as you do the same thing to both sides of the equation. However, in this case you want to find the value of 'x', i.e. 'x' on it's own, not 'x' multiplied by -3, so you simply divide both sides of the equation by -3 to find the value of 'x'.
For example, say you had equation: 6x = 24
If you now wanted to find the value of 'x' you would divide both sides of the equation by 6. So you would get:
6x/6 = 24/6
So x = 4
Hope that helped, and feel free to ask more if you still have questions. :)