Yeah, that's a tough one.  The correct answer is 5,879.56.  To add or subtract decimal numbers you must line up the decimal points first then fill in any blank spots with zeros.

I recommend reading Volume 1 of the Learn Math Fast System.  That book will give you step-by-step instructions on how to solve problems like that one.  It is written for all ages at a fourth-grade reading level, and older students can complete it in about a month.  Younger students will need more time.