midpoint means the segments beside it aer equal measure
therefor
JK=KL
7x+4=12x-21
minus 7x both sides
4=5x-21
add 21 both sides
25=5x
divide oth sides by 5
5=x
LJ=2(JK)=2(KL)
find either JK or KL
7x+4
7(5)+4
35+4
39=JK
double taht
39*2=78
JL=78 units