>How do I make X the subject, if 'X' appears on both sides of the '='

You have to find a way to get all the x's on the same side. Some forms are easier than others.

>... for example 2X = 6X over 5Z + 8Z

That is (using lower case)

2x = 6x/(5z + 8z)

Well first of all those terms in the bottom can be combined.

2x = 6x/13z

Generally when you have a fraction, you're going to want to multiply by the denominator so it's not a fraction any more. So multiply both sides by 13z.

26xz = 6x

Now, finally, you can move all the x terms to the same side. Let's subtract the 6x from both sides so you no longer have an x on the right.

26xz - 6x = 0

And you can factor out an x

x(26z - 6) = 0

And you can divide both sides by the thing in parentheses

x(26z - 6)/(26z - 6) = 0/(26z - 6)

x = 0

To summarize, when you have fractions involved, it's often this process

* Clear fractions by multiplying by the denominator

* Move all the x terms to the same side

* Factor out the x

* Divide by the thing multiplying x

Feel free to ask with more complicated examples.