How do you simplify x^2-(3x-5x^2)?

$- \left(3 x - 5 {x}^{2}\right) = - 3 x + 5 {x}^{2}$
After all, the minus represents a moltiplication by $- 1$
$- \left(3 x - 5 {x}^{2}\right) = \left(- 1\right) \left(3 x - 5 {x}^{2}\right)$
x^2-(3x−5x^2)=x^2-3x+5x^2, and adding the two terms involving ${x}^{2}$, we have ${x}^{2} - 3 x + 5 {x}^{2} = 3 x + 6 {x}^{2}$.
If you want, you can rewrite it as $3 x \left(1 + 2 x\right)$ (I don't know which versions you consider as more simplified)