Firstly, to expand an equation like this, you must multiply the brackets by the number outside of the brackets. Make sure that you multiply every number inside the bracket by the number directly outside, and remember the signs:

3(2x + 5) - 2(x-4) becomes

(3 x 2x) + (3 x 5) + (-2 x x) + (-2 x -4) = 

6x + 15 - 2x + 8 (remember that '-' x '-' = '+')

Then you need to do something called 'collecting the like terms'. This means collecting together all the 'x' terms and all of the 'number' terms, like this:

6x + 15 - 2x + 8 becomes

(6x - 2x) + (15 + 8), working this out means the answer is:

4x + 23