What is 0.0039 written in scientific notation?

3.9 * 10^-3. The way you do it is to make the number less than 10 but more than 0, then make it a product of a power of 10, ie: 120 = 1.2 * 10^2. And for numbers less than zero, the power is negative, ie: 0.03 = 3 ^ 10*-2