$\text{a)}\left(-1\frac{3}{4}\right)\cdot \left(-2,5\right)\cdot 3\frac{5}{6}\cdot \left(-6\right)\cdot \frac{4}{7}\cdot 2=$  

$=\left[\left(-1\frac{3}{4}\right)\cdot \frac{4}{7}\right]\cdot \left[\left(-2,5\right)\cdot 2\right]\cdot \left[3\frac{5}{6}\cdot \left(-6\right)\right]=$  

$=\left(-\frac{{\sout{7}}^1}{{\sout{4}}^1}\cdot \frac{{\sout{4}}^1}{{\sout{7}}^1}\right)\cdot \left(-5\right)\cdot \left[\frac{23}{{\sout{6}}^1}\cdot \left(-{\sout{6}}^1\right)\right]=$  

$=-1\cdot \left(-5\right)\cdot \left(-23\right)=5\cdot \left(-23\right)=-115$