Since length*width=area, (x+12)*width=x²-144. Dividing (x+12) from both sides, we get (x²-144)/(x+12)=width. One formula we can use is that
(a²-b²)=(a-b)*(a+b). Since 144=12², that means that x²-144=(x-12)*(x+12). Plugging that in, we get (x-12)*(x+12)/(x+12)=(x-12)=width
