To apply the property of negative integer exponents, we can use the rule that states \( a^{-n} = \frac{1}{a^n} \).
For \( 7^{-3} \):
\[ 7^{-3} = \frac{1}{7^3} \]
Now, we calculate \( 7^3 \):
\[ 7^3 = 7 \times 7 \times 7 = 343 \]
Thus, substituting back, we have:
\[ 7^{-3} = \frac{1}{7^3} = \frac{1}{343} \]
So, the final answer is:
\[ 7^{-3} = \frac{1}{343} \]