a) \(-12:\left(\frac{3}{4}-\frac{5}{6}\right)^2=-\frac{12}{\left(\frac{3}{4}-\frac{5}{6}\right)^2}=-\frac{12}{\frac{1}{12^2}}\)
\(=-\frac{12^2.12}{1}=12^2.12\)
\(=-12^3=--1728\)
b) \(\frac{7}{23}.\left[\left(-\frac{8}{6}\right)-\frac{45}{18}\right]=\frac{7}{23}.\left(-\frac{4}{3}-\frac{5}{2}\right)\)
\(=\frac{7}{23}.\left(-\frac{23}{6}\right)=-\frac{7}{23}.\frac{23}{6}\)
\(=-\frac{7}{6}\)