I would use t with 39 degrees of freedom which is close to the z-score of 1.96, but not the same.

Playing with Free p-Value Calculator for the Student t-Test

I get 2.0227 as

AND you should use t since this problem assumes we do not know the population variance.

BUT I bet the problem wants 1.96 instead.

anyhow, to do your problem, you shoudl take the difference of each interval.

83.09 - 65.83 =

I would go with 83.09 - 65.83 =

but most likely you're being asked to obtain s via 83.09 - 65.83 =