Amount financed (m) = $2,500 number of payments per year (y) = 12 number of payments (n) = 36 total interest (c) = $158.15

The number of payments per year is 12
The number of payments is 36 which means that the term lasted for 3 years.
The total interest is $158.15 and the amount financed is $2500.
The interest rate is
158.15/2500 = 6.33%
In nominal month, that is
(1 + r/12)^12 - 1 = 0.0633
Solve for r