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%