We have from Thales theorem that because the 2 triangles are similar, JM/JK=JN/JL. We have that JN=x, JL=x+24 and that JM=7 and JK=28. Substituting, we get:
x/(x+24)=7/28.
Hence, x/(x+24)=1/4. This leads to 4x=x+24, 3x=24, x=8. We have solved for x=JN and this is equal to 8.