Yes -6x can represent a positive number, as well as a negative number. What you have to keep in mind is that x is a variable, meaning that any number can replace the x. To get a positive number from -6x all you have to do is insert a negative number, that could be -1, -2, -5, -100, etc, because a negative cancels a negative. (-6)*(-2)=12, or you can put in a positive number to get a negative answer (-6)*(2)=-12. The parenthesis are optional, I only typed it here so you can better see what I'm doing, hopefully.
Basically remember, when dealing with two numbers:
+ * + = +
+ * - = -
- * - = +