To convert a number without the use of the expression 10^-4 you would have to treat it like an integer line for instance on an integer line the negative is to the left and the positive number is to the right so 3.91*10^-4 means that the decimal would have to move 4 places to the left so the answer would be .000391 instead of 3.91*10^-4.
I hope this helps you :)
