To subtract the fractions \( \frac{3}{10} - \frac{2}{7} \), we first need a common denominator.
The least common multiple (LCM) of 10 and 7 is 70.
Next, we convert each fraction to have a denominator of 70:
\[ \frac{3}{10} = \frac{3 \times 7}{10 \times 7} = \frac{21}{70} \]
\[ \frac{2}{7} = \frac{2 \times 10}{7 \times 10} = \frac{20}{70} \]
Now we can subtract the two fractions:
\[ \frac{21}{70} - \frac{20}{70} = \frac{21 - 20}{70} = \frac{1}{70} \]
So, \( \frac{3}{10} - \frac{2}{7} = \frac{1}{70} \).