$a)-21-7=-28$

$-18+6=-12$

$-14-\left(-23\right)=-14+23=23-14=9$

 

$b)-6\cdot 7=-42$

$-8\cdot \left(-9\right)=72$

$-75:3=-25$

 

$c)\frac{3}{5}+2\frac{2}{5}=2\frac{5}{5}=2+1=3$

$5\frac{9}{11}+\frac{6}{11}=5\frac{15}{11}=5+1\frac{4}{11}=6\frac{4}{11}$

$1\frac{4}{7}+2\frac{5}{7}=3\frac{9}{7}=3+1\frac{2}{7}=4\frac{2}{7}$

 

$d)10-\frac{1}{3}=9\frac{3}{3}-\frac{1}{3}=9\frac{2}{3}$

$12\frac{1}{5}-\frac{4}{5}=11\frac{6}{5}-\frac{4}{5}=11\frac{2}{5}$

$3\frac{1}{7}-4=-\frac{6}{7}$

 

$e)\frac{2}{11}\cdot 5=\frac{10}{11}$

$\frac{5}{8}\cdot \frac{2}{3}=\frac{5}{4}\cdot \frac{1}{3}=\frac{5}{12}$

$\left(-\frac{2}{3}\right)\cdot \left(-\frac{3}{8}\right)=\frac{2}{3}\cdot \frac{3}{8}=\frac{2}{8}=\frac{1}{4}$

 

$f)\frac{2}{5}:3=\frac{2}{5}\cdot \frac{1}{3}=\frac{2}{15}$

$\frac{3}{7}:\frac{1}{2}=\frac{3}{7}\cdot \frac{2}{1}=\frac{6}{7}$

$-\frac{5}{8}:\frac{1}{3}=-\frac{5}{8}\cdot \frac{3}{1}=-\frac{15}{8}=-1\frac{7}{8}$

 

$g)0,8+0,4=1,2$

$1,2-0,7=0,5$

$2,1-3=-0,9$

 

$h)0,3\cdot 2=0,6$

$1,6\cdot \left(-3\right)=-4,8$

$0,4:2=0,2$