Add. \[ \left(3 x^{2}-5 x+4\right)+\left(-2 x^{2}+9 x-9\right) \] \[ \left(3 x^{2}-5 x+4\right)+\left(-2 x^{2}+9 x-9\right)= \]

Step 1 ：The problem is asking to add two polynomials: \(3 x^{2}-5 x+4\) and \(-2 x^{2}+9 x-9\).

Step 2 ：To add polynomials, we combine like terms. Like terms are terms that have the same variable and exponent.

Step 3 ：In this case, the like terms are \(3x^2\) and \(-2x^2\), \(-5x\) and \(9x\), and \(4\) and \(-9\).

Step 4 ：We add the coefficients of the like terms to get the sum: \(x^2 + 4x - 5\).

Step 5 ：The sum of the two polynomials is \(\boxed{x^{2}+4 x-5}\).