insert one dash where needed. submit your best essay for a chance to be featured in our monthly magazine and…

Brief Explanations:

The dash should be used to set off an appositive or an explanation. Here, "a trip for two to Hawaii" is an explanation of the "grand - prize", so a dash should be inserted between "prize" and "a".

Answer:

Submit your best essay for a chance to be featured in our monthly magazine and win the grand prize - a trip for two to Hawaii.