If K is the midpoint of JL, then by definition JK = KL.
Set up an equation:
12x -21 = 7x + 4
subtract 7x from both sides
5x - 21 = 4
add 21 to both sides
5x = 25
divide both sides by 5
x = 5
substitute 5 back in to either segment length then double to get the length of JL.
JK = 7(5) + 4
JK = 35 + 4
JK = 39
JL = 2(39)
JL = 78
