a: \(-81\cdot36+\left(-81\right)\cdot63-81\)
\(=36\cdot\left(-81\right)+63\cdot\left(-81\right)+\left(-81\right)\)
\(=-81\left(36+63+1\right)\)
\(=-81\cdot100=-8100\)
b: \(23\cdot\left(-38+123\right)-123\cdot\left(23-38\right)\)
\(=-23\cdot38+23\cdot123-123\cdot23+123\cdot38\)
\(=123\cdot38-23\cdot38=38\left(123-23\right)=38\cdot100=3800\)