Explanation:
I am not sure what the actual details of the problem are, and what
4x + 6
7x + 15 are.
are the number of dashes ("-") an indicator ?
then the whole distance JL = 138 = 4+6 = 10 distance units.
1 distance unit is then 138/10 = 13.8
JK is then 4 distance units = 4×13.8 = 55.2
KL is the other 6 distance units = 6×13.8 = 82.8
or did the dashes not mean anything ?
and the 2 expressions stood for the 2 segment lengths ?
then it was just
JK = 4x + 6
KL = 7x + 15
and
JK + KL = JL
4x + 6 + 7x + 15 = 138
11x + 21 = 138
11x = 117
x = 117/11 = 10.63636363...
JK = 4x + 6 = 4×117/11 + 6 = 48.54545454... ≈ 48.55