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