(a) \( \left(3 x^{2}-x+7\right)-\left(x^{2}-5 x+4\right) \)

Let’s simplify that expression step by step. First, distribute the negative sign in front of the second bracket: \[ (3x^2 - x + 7) - (x^2 - 5x + 4) = 3x^2 - x + 7 - x^2 + 5x - 4 \] Now, combine like terms: 1. For \(x^2\) terms: \(3x^2 - x^2 = 2x^2\) 2. For \(x\) terms: \(-x + 5x = 4x\) 3. For constant terms: \(7 - 4 = 3\) Putting it all together, we get: \[ 2x^2 + 4x + 3 \]