Answer:
The budget line will shift outward on the horizontal axis.
Step-by-step explanation:
One of the laws of the demand is that the lower the price of a good, the higher the quantity of that good that is purchased.
From the question, a decline in the price of X from $9 to $6, will lead to an increase in the quantity of X that is bought.
Since the price of Y still remains at $4, if the price of X now declines to $6, the budget line will shift outward on the horizontal axis.