There are 48 monthly payments. You can use the formula for the Present Value of an annuity:

$12000 = 300 \frac{1}{i/12}[ 1-\frac{1}{(1+i/12)^{48}}]$

to find the interest rate

However there is no explicit solution for i, it is solved by trial and error. The value I get is 9.2428%

There are 48 monthly payments. You can use the formula for the Present Value of an annuity:

$12000 = 300 \frac{1}{i/12}[ 1-\frac{1}{(1+i/12)^{48}}]$

to find the interest rate

However there is no explicit solution for i, it is solved by trial and error. The value I get is 9.2418%