How do I solve this equation? 3x(4b-2x)

Well, I don't think this is an equation. An equation has an '=' sign. 

What you wrote is actually an expression.

Now you can't really solve this thing because there is no = sign.

But if the question told you to simplify the expression...

Distribute 3x and get:

3x*4b - 3x*2x.

Simplify to get 12bx-6x^2.

And that would be the answer if the question said "simplify the expression".