$3{x}^{2}-5x+7-\left( -2+6x-7{x}^{2} \right)$
When there is a $-$ in front of an expression in parentheses, change the sign of each term of the expression and remove the parentheses$3{x}^{2}-5x+7+2-6x+7{x}^{2}$
Collect like terms$10{x}^{2}-5x+7+2-6x$
Collect like terms$10{x}^{2}-11x+7+2$
Add the numbers$10{x}^{2}-11x+9$