To multiply \( \frac{2}{7} \) by 21, you can simply multiply the numerator (2) by 21 and keep the denominator (7) the same:
\[ \frac{2 \times 21}{7} = \frac{42}{7} \]
Now, divide 42 by 7:
\[ \frac{42}{7} = 6 \]
Therefore, \( \frac{2}{7} \times 21 = 6 \).