Net worth is equal to assets minus liabilities. which event will have the greatest impact (positive or negative) on one's net worth?

The answer would be: Go on a vacation that costs $3,500 

Paying off money for buying a car will not decrease your net worth as you get the car as assets for the money you use. But the depreciates 20% will cause you to lose $3,000 assets. Assuming you are not buying assets at all, go on a vacation that costs $3,500 will increase liability without any effect on assets. Paying up bills will decrease your asset but it also decreases your liability so the net worth wouldn't change.