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