JL and MK have the same value, because they link heights and lengths with the same proportion: Height KL = Height JM ; Length ML = Length JK. So you can conclude that you can do:
JL = MK
-> 2x + 5 = 7x - 40
-> 5x = 45
-> x = 45/5
-> x = 9
MK = ?
-> MK = 7x - 40
-> MK = 7.(9) - 40
-> MK = 63 - 40
-> MK = 23