This absolute value equation is


    |x -L| = 0.0012*L.


Here x is the length of the steel bar at temperature 400°C  (= 100°C above 300°C),

       or the length of the steel bar at temperature 200°C  (= 100°C below 300°C);


L is the length of the bar at 300°C (designation introduced in the condition);



and 0.0012 = 0.12%.