You compare the average bicycle fatalities in states with and without the helmet law in place. You observe states with more fatalities are less likely to have the helmet law. Is this convincing evidence of a causal link?

it's a convincing evidence of a casual link

Explanation:

It's recorded that 97% of accidents that affected bicyclist greatly were because they weren't putting on their helmet. A state ignoring putting laws to implement the enforcement of the helmet rule is proving a convincing evidence of a casual link because if the rules are enforced fatality won't be much.