Answer:
Any number other than 2
Explanation:
Well if we plug in any number other than 2 like 1 for the blank and distribute,
the right side becomes,
3x + 6 = 3x + a number other than 6
Parallel lines have the same slope but different y intercept.
Look at the image below ↓
Thus,
to make the equation have no solution the blank has to be any number other than 2.
Hope this helps :)