Let x be the salary. If x is more than the average, we want
[tex]x-34100<2570[/tex]
If x is less than the average, we want
[tex]34100-x<2570[/tex]
The two conditions can be summarized into
[tex]|x-34100|<2570[/tex]
In fact, the absolute value of the difference of two numbers tells you "how far" those two numbers are, without taking into account which of the two is bigger.