To rewrite \( 15^{-38} \) as an expression with positive exponents only, we can use the property of negative exponents, which states that:
\[ a^{-n} = \frac{1}{a^n} \]
Applying this property:
\[ 15^{-38} = \frac{1}{15^{38}} \]
Thus, the expression \( 15^{-38} \) can be rewritten as:
\[ \frac{1}{15^{38}} \]