First off, let's pick a variable.
Since the number of years is unknown, that can be made into our "x".
Now then: we know that the level is currently 72 inches below sea level. It sinks two inches each year.
Those two pieces of information tell us (a) the starting point and (b) the rate at which the level drops.
Since the rate is -2 inches per year, this can be represented by -2x.
The 72 inches is the beginning point, however since it is 72 inches UNDER sea level, we need to make this a negative.
Now we combine the two in order to create the expression of:
-72 - 2x
where x is the years of which New Orleans has been sinking.
