a: \(A=\dfrac{-\left(x+1\right)}{x-3}+\dfrac{2x-1}{x+3}-\dfrac{x\left(x-1\right)}{\left(x-3\right)\left(x+3\right)}\)
\(=\dfrac{-x^2-4x-3+2x^2-6x+x-3-x^2+x}{\left(x-3\right)\left(x+3\right)}\)
\(=\dfrac{-8x-6}{\left(x-3\right)\left(x+3\right)}\)
b: \(B=\dfrac{5}{2x\left(x+3\right)}+\dfrac{3x^2-4}{\left(x-3\right)\left(x+3\right)}-3\)
\(=\dfrac{5\left(x-3\right)+2x\left(3x^2-4\right)-6x\left(x+3\right)\left(x-3\right)}{2x\left(x+3\right)\left(x-3\right)}\)
\(=\dfrac{5x-15+6x^3-8x-6x\left(x^2-9\right)}{2x\left(x+3\right)\left(x-3\right)}\)
\(=\dfrac{6x^3-3x-15-6x^3+54x}{2x\left(x+3\right)\left(x-3\right)}\)
\(=\dfrac{51x-15}{2x\left(x+3\right)\left(x-3\right)}\)
