a: \(=\left(\dfrac{-1}{2}+\dfrac{3}{4}\right):\left(\dfrac{9}{16}+1\right)=\dfrac{-1}{4}:\dfrac{25}{16}=\dfrac{-1}{4}\cdot\dfrac{16}{25}=\dfrac{-4}{25}\)
b: \(B=\dfrac{5}{7}\cdot\dfrac{4}{9}+\dfrac{1}{7}-\dfrac{23}{7}\cdot\dfrac{4}{9}=\dfrac{4}{9}\cdot\left(\dfrac{5}{7}-\dfrac{23}{7}\right)+\dfrac{1}{7}=\dfrac{4}{9}\cdot\dfrac{-18}{7}+\dfrac{1}{7}=-1\)