We know that the items cost $2900 at the end of the loan you will have to pay $3600.
The simple interest formula is given as:

where P is the principal, r is the interest rate.
In this case we have:

With this we can conclude that:
The finance charge is $700.
The interest rate over the total loan is 2.4% (this means that per month we are going to pay an interest rate of 0.066%).