To find the value that makes the equation undefined, we need to identify when the denominator equals zero.
The equation you've provided can be interpreted as:
\[ \frac{x + 3}{2x + 4} = 18 \]
The denominator is \(2x + 4\). To find when the fraction is undefined, we set the denominator equal to zero and solve for \(x\):
\[ 2x + 4 = 0 \]
Subtract 4 from both sides:
\[ 2x = -4 \]
Now, divide both sides by 2:
\[ x = -2 \]
Thus, the value that makes the equation undefined is \(\boxed{-2}\).