This is the concept of algebra, we are required to find the value of x given that point K is on line segment JL, KL =x-12, JL=36 inches, JK=42+x. To obtain tha value of x we proceed as follows;
JL=KL+JK
substituting the value we get:
36=(x-12)+(x+42)
36=x-12+x+42
36=x+x+42-12
36=2x+30
2x=36-30
2x=6
hence;
x=3