"3 miles below sea level" may be written as -3. The negative indicates you are below sea level. Going above sea level means the value would be positive.
"earning 45 dollars" can be written as +45 or simply 45. If you lost 45 dollars, then it would be -45. If you are in debt 45 dollars, then it would be -45.
"moving back 5 spaces" is represented by -5 whereas moving forward 5 spaces is +5 or simply 5.