Well, you can take 3(x2-4x) - 5(2x2-x) and make it a lot simpler.
Start by multiplying 3 by everything inside the first brackets.
(3x²-12x) - 5(2x²-x)
Then do the same to the other side:
(3x²-12x) - (10x²-5x)
From here, we see that there are two similar sides of the equation. As well, we don't need the brackets anymore because there is no multiplication or division.
3x² - 12x - 10x² + 5x
We turn the minus on the right to plus because of the brackets in the first place. You can think of "a - (b - c)" as "a + -1(b - c)" in which case you multiply -1 to b and - c. So the minus becomes plus. It's sort of something you just have to trust yourself on if you still don't get it.
Lastly, combine the terms.
3x² - 10x² = -7x²
and
12x + 5x = 17x
so
-7x² - 17x
