Answer:
Positive; Subsidy for
Negative; Tax on
Explanation:
An externality is the economic term that describes a cost or benefit incurred or received by a third party, over which said party has no control over its development.
When externalities result in a benefit for a certain group they are said to be positive and when it is detrimental it is said to be negative.
Taking public transportation in a crowded city is benefitial to drivers making this a positive externality. In order to encourage people to take public transportation, a subsidy for those who take transportation may be adopted.
On the other hand, people who choose to drive their own car impose a negative externality and may encourage a tax on those who drive their own car.