Usually, the decision about whether or not to use scientific notation is up to you. An answer of 1.2×10³ is equivalent in EVERY WAY to 1200. Thus, you can answer either way. But be sure to check to see if the question states how the answer needs to be given.

There is one condition under which scientific notation MUST be used, however. That is when you need to make one or more zeros significant. For example, suppose you solve a problem and the answer is 100.11, and the rules for significant figures require you to report the answer to 2 significant figures. You can’t write 100, because it has only 1 significant figure. Thus, you HAVE to write 1.0×10² to make one of the zeros significant.