Question

7 to the power of -20 - 7 to the power of-21

Answer 1

$7^{-20}-7^{-21}\\ \\= \frac{1}{7^{20}} - \frac{1}{7^{21}}\\ \\= \frac{7}{7^{21}} - \frac{1}{7^{21}}\\ \\= \frac{1}{7^{21}} \times (7-1)\\ \\ =6 \times \frac{1}{7^{21}}\\ \\ =\frac{6}{7^{21}}$