a: \(\left(\dfrac{2}{5}:\dfrac{2}{3}+6\right)\cdot3-18\)
\(=\left(\dfrac{2}{5}\cdot\dfrac{3}{2}+6\right)\cdot3-18\)
\(=\left(\dfrac{3}{5}+6\right)\cdot3-18\)
\(=\dfrac{9}{5}+18-18=\dfrac{9}{5}\)
b: \(25\cdot\left(-37\right)+25\cdot\left(-63\right)\)
\(=25\left(-37-63\right)\)
\(=25\cdot\left(-100\right)=-2500\)