This is the concept of algebra, given that K is the midpoint of JL, it implies that JK=KL. Therefore formulating an equality we shall have:
JK=KL
7x+4=12x-21
this can be written as:
12x-21=7x+4
collecting like terms;
12x-7x=4+21
5x=25
dividing both sides by 5 we get:
x=5
therefore:
JK=7x+4
JK=7*5+4=39
But,
JK=LK
therefore
JL=39*2=78