To determine where the equation \( \frac{6x}{8x - 5} = 12 \) is undefined, we need to identify when the denominator is equal to zero. The term that helps us determine this is \( 8x - 5 \).
Set the denominator equal to zero:
\[ 8x - 5 = 0 \]
Now, solve for \( x \):
\[ 8x = 5 \] \[ x = \frac{5}{8} \]
Therefore, the equation is undefined when \( x = \frac{5}{8} \).
The final answer is:
\( x \neq \frac{5}{8} \).