The absolute value of any number is written as:
[tex]\lvert{x}\rvert[/tex]On the line, the distance between two numbers x and c (or any other two numbers), is express as the absolute value of their difference, that is:
[tex]\lvert{x-c}\rvert[/tex]Now that we have the distance between x and c we just need to express that this distance is less than d units, we achieve this by using the inequality sign <, that is:
[tex]\lvert{x-c}\rvertTherefore, the expression we are looking for is:[tex]\lvert{x-c}\rvert