Since point K is the midpoint of J and L, then JK and KL are equal to each other. You solve that equation which is 7x+4 = 12x-21. You would get x = 5. Now, JK and KL added together would equal JL, so you plug in x =5 for each expression, then add them up to get 78 as your final answer.
JL = 78 units long.