$a)\left(-\frac{3}{8}\right)\cdot 6,4-4,24:0,8=-\frac{3}{{\sout{8}}^1}\cdot \frac{{\sout{64}}^8}{10}-42,4:8=-\frac{24}{10}-5,3=-2,4-5,3=-7,7$  

$b){\left(-\frac{5}{6}\right)}^2:2\frac{1}{4}+3\frac{1}{3}:\left(-1\frac{1}{8}\right)=\frac{25}{36}:\frac{9}{4}+\frac{10}{3}:\left(-\frac{9}{8}\right)=\frac{25}{{\sout{36}}^9}\cdot \frac{{\sout{4}}^1}{9}+\frac{10}{3}\cdot \left(-\frac{8}{9}\right)=$