Simplify 3 (2x+5) - 4 (2x-7)

For this you need to expand the brackets first:

3 (2x+5) - 4 (2x-7) = 6x+15-8x-28

And then now you need to add and simplify:

6x+15-8x-28 - > 6x-8x = - 2x

15-28 = - 13

So that leaves

-2x - 13