Since K is the midpoint of JL, it divides the segment JL into two equal halves.
That means ⇒ JK = KL
Substitute expressions for the parts ⇒ 7x + 4 = 12x - 21
Now solve the equation: 7x - 7x + 4 = 12x - 7x - 21
4 = 5x - 21
4 + 21 = 5x - 21 + 21
25 = 5x
5 = x
Substitute the value into one of the expressions: JK = 7(5) + 4 = 39
JL = JK + KL
JL = 39 + 39 = 78