Given the ratio of vinegar to baking soda is 3 to 1, for every 3 parts of vinegar, there is 1 part of baking soda.
If there are 6 parts of baking soda, we can set up the following relationship based on the ratio:
Let \( v \) be the parts of vinegar. According to the ratio:
\[ \frac{v}{6} = \frac{3}{1} \]
Cross-multiplying gives:
\[ v = 6 \times 3 = 18 \]
Thus, you need 18 parts of vinegar.
The correct response is 18.