The Minus Reaches Everything

The same rule works when there are more terms inside the parentheses. In $2x^{2}-\left(x^{2}-4x+1\right)$, a trinomial is being subtracted, so all three of its terms change sign: $2x^{2}-x^{2}+4x-1$.

Do not forget the last term. The constant 1 at the end is inside the parentheses too, so it changes to $-1$. Count the terms inside before you start, and then check that you changed that many signs.

Collect the like terms that remain. The squared terms give $2x^{2}-x^{2} = x^{2}$. The $4x$ and the $-1$ have no like terms, so they stay as they are. The result is $x^{2}+4x-1$.