**Solution**

L.C.M. of the denominators i.e., 21 and $12=84$

$\therefore \quad \frac{5}{21}=\frac{5 \times 4}{21 \times 4}=\frac{20}{84}$

$\& \quad \frac{7}{12}=\frac{7 \times 7}{12 \times 7}=\frac{49}{84}$

$\therefore \quad$ The required fractions are $\frac{20}{84}$ and $\frac{49}{84}$; Ans.