Scientific notation is a way of writing very big or small numbers. In scientific notation, a number can look like “−2 * 10−3”. To write this the usual way, you take the first number (−2) and move the decimal point the number of places given by the superscript (−3). (The 10 is always there; you can ignore it.) Of course, −2 was written without a decimal point, but it's the same thing as −2., so we move the decimal point of −2. three places to the left. (If we had 3 instead of −3, we would move it three places to the right.) Moving one place gives −0.2, moving two places gives −0.02 and moving three places gives −0.002, so that's our answer.
If the exponent is 0, then you don't need to move the decimal point at all! So −8.6 * 100 is just −8.6.