Martini, algebra isn't that hard; basically you just need to try and get X or N or whatever all by itself, and use whatever means at your disposal to do so.
The basic rule of thumb is: If you do a mathematical operation to one side of the equation, you must do it to the other. For example, in the case of:
x+3 = 2x-5
If you want to add 5 to the right side, you need to do it to the left side too, so that they remain equal.
x+3+5 = 2x-5+5
x+8 = 2x
The reason I did that was to get one side of the equation to get x by itself. Now at this point, I can just subtract x from both sides (since x is a number),
x-x+8 = 2x-x
8 = x
x = 8
The more you practice, the quicker this becomes second nature.