a. 

$\left ( \frac{1}{4}+\frac{1}{7} \right )\times \frac{1}{3}=\left ( \frac{7}{28}+\frac{4}{28} \right )\times \frac{1}{3}=\frac{11}{28}\times \frac{1}{3}=\frac{11}{84}$

$\left ( \frac{1}{4}+\frac{1}{7} \right )\times \frac{1}{3}=\frac{1}{4}\times \frac{1}{3}+\frac{1}{7}\times \frac{1}{3}=\frac{1}{12}+\frac{1}{21}=\frac{7}{84}+\frac{4}{84}=\frac{11}{84}$

b.

$\left ( \frac{1}{4}-\frac{1}{7} \right )\times \frac{1}{3}=\left ( \frac{7}{28}-\frac{4}{28} \right )\times \frac{1}{3}=\frac{3}{28}\times \frac{1}{3}=\frac{3}{84}$

$\left ( \frac{1}{4}-\frac{1}{7} \right )\times \frac{1}{3}=\frac{1}{4}\times \frac{1}{3}-\frac{1}{7}\times \frac{1}{3}=\frac{1}{12}-\frac{1}{21}=\frac{7}{84}-\frac{4}{84}=\frac{3}{84}$