A prefix is located before the root of a word. So once you take away all the prefixes and suffixes, you are only left with the root, or the part of the word that cannot be separated any longer. So here, the root of the word engage is gage, which means that the prefix is en-. You always write a dash after the prefix (en-) to show that it is a prefix and that the remainder of the word should follow. This word can have another prefix, such as dis- in disengage.
