To clear fractions in $\dfrac{x}{7} - \dfrac{4}{5} = \dfrac{6}{7} - \dfrac{2}{5}$ we may multiply both sides by—

To clear the fractions, multiply by the least common multiple of the denominators. The denominators are $7$ and $5$, whose LCM is $35$, so multiplying by $35$ removes every fraction at once.