You can usually translate below to mean the number is minus or at least smaller than something else. In this case it is minus. So the answer is found as
535 - - 8 = 543 feet. The first minus sign means difference and the second means below sea level. (535 is feet above sea level). This is one of those cases where it makes sense to say that a minus times a minus is a plus.
Let's try this a slightly different way. Suppose there is a hill above a gully. The hill is 30 feet above the gully. The gully has a bottom that is 8 feet below where the hill begins. If you wanted to walk from the top of the hill to the bottom of the gully, how far would you have to go? Never mind all the plus and minus stuff. Just draw the picture, and see what happens. Or imagine you are doing the walking.
Wouldn't you have to go 30 + 8 feet? Isnt' that 38 feet? Same with your Louisiana problem. It (the Louisiana problem) is just more mathematical. And getting two minus signs to equal a plus is no easy trick.
Be sure you understand the answer to this problem. It is one of the cornerstone's of Algebra.