Here's how we get that:

1. Move the decimal point to the right until you have a number between 1 and 10. In this case, we move it six places to the right to get 4.2.

2. Count the number of places you moved the decimal point. Since we moved it six places to the right, the exponent is -6.

3. Combine the number and the exponent to get the scientific notation: 4.2 x 10^-6.