8 = 18 - 2x
First we should note that we want the X term by itself. To do that we must first get rid of any values besides -2x on that side of the equation. 
So, we remove 18 by subtracting 18 on BOTH sides of the equation. 
Leaving our equation looking like this: 
8 - 18 = -2x
Simplify the 8 - 18 by subtracting the two. 
Leaving you with this: 
-10 = -2x
Remove the -2 from -2x by dividing -2 on both sides. 
-10/-2 = x
Simplify (Remember a double negative equals a positive)
5 = x
So! X = 5