To rewrite the expression \( 6^3 \cdot 6^5 \) using a single exponent, you can use the property of exponents that states:
\[ a^m \cdot a^n = a^{m+n} \]
In this case, \( a = 6 \), \( m = 3 \), and \( n = 5 \). Applying the property:
\[ 6^3 \cdot 6^5 = 6^{3+5} = 6^8 \]
So, the expression can be simplified to:
\[ 6^8 \]