To rewrite \( 15^{-38} \) using only positive exponents, you can apply 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 \( 15^{-38} \) rewritten with positive exponents only is:
\[ \frac{1}{15^{38}} \]