To find the asymptote of the function \( h(x) = 2 \log_8(x - 3) \), we need to identify the values of \( x \) for which the function is undefined.
The logarithmic function is only defined for positive arguments, so we need to set the argument of the logarithm greater than zero:
\[ x - 3 > 0 \]
Solving this inequality gives:
\[ x > 3 \]
Thus, the function \( h(x) \) is defined for \( x > 3 \) and is undefined for \( x \leq 3 \). This means that there is a vertical asymptote at:
\[ x = 3 \]
So, the correct response is:
x = 3.