You have the formula incorrectly stated. You can read up on the diode equation in the AAC tutorial section of this website. hgmjr
HINT: With a circuit made of a diode and a 3K resistor in series with 12 volts applied across the combination then in the case of a short circuit across the diode, the current that would result would be 12/3K. This current computes to 4 milliamps. The answer for the current will be somewhat less than 4 milliamps but not as small as 65 nanoamps as you have erroneously computed. hgmjr
What you have here is a transcendental function. Such a function is often solved either graphically or by using a numerical analysis method. Since you have been asked to provide an accuracy to the 4th decimal place then there is a high probability that you are being asked to apply your knowledge of numerical analysis to solve the problem. Was a particular numerical analysis method specified as part of the homework assignment? hgmjr
Another Hint: You already have one expression for the current flowing in the diode. Now you need another one. You know the total voltage applied across the diode and the 3000 ohm series resistor. Assume that the voltage across the diode is x, then using the 3000 ohm and the 12Vdc you can form the expression for the current flowing in the 3000 ohm resistor as a function of the unknown diode voltage x. You now have two expressions for the current flowing through the diode and the 3000 ohm resistor. Set them equal to each other and then you will notice the equality that is formed does not yield to ordinary algebraic manipulation. This is probably a good point at which to consult your textbook to see if there is an example problem that can help you with the next step in the process of finding a solution. hgmjr
Those are consistent with the values I obtained when I used the 0.025mv for the values of Vt. Would you mind telling us what steps you used to get to the answer? hgmjr