$a)$

$5\cdot \left(\frac{1}{7}+\frac{3}{5}\right)=5\cdot \left(\frac{5}{35}+\frac{21}{35}\right)={\sout{5}}^1\cdot \frac{26}{{\sout{35}}^7}=\frac{26}{7}=3\frac{5}{7}$

$\frac{5}{7}+3=3\frac{5}{7}$