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

i = %

  I ≈ 4.0248%

Step-by-step explanation:

The total amount repaid is ...

  $2500 +158.15 = $2658.15

so the monthly payment is

  $2658.15/(36 mo) = $73.8375/mo

___

There is no analytic method by which the interest rate can be calculated, so it must be found by other means, typically by a financial calculator or graphing calculator. The attached shows a graphing calculator result for the interest rate necessary to obtain the above monthly payment, along with a refinement of the result using Newton's Method iteration.

The annual interest rate must be about 4.0248% in order for the total interest on that loan to be $158.15.