1. **State the problem:** Perform the operation and simplify the expression: $$\frac{7}{21} - \frac{5}{21}$$ 2. **Formula and rules:** When subtracting fractions with the same denominator, subtract the numerators and keep the denominator the same: $$\frac{a}{c} - \frac{b}{c} = \frac{a-b}{c}$$ 3. **Apply the formula:** $$\frac{7}{21} - \frac{5}{21} = \frac{7-5}{21} = \frac{2}{21}$$ 4. **Simplify the fraction:** Check if numerator and denominator have common factors. The numerator is 2 and denominator is 21. Since 2 and 21 have no common factors other than 1, the fraction is already in simplest form. 5. **Final answer:** $$\frac{2}{21}$$ This matches the multiple-choice option 2/21.