JK and KL make up line JL.
JL = 2x + 8, JK = 2x - 2, KL = x - 9
If JK and KL make up line JL:
JK + KL = JL
We are solving for KL, so isolate it.
JK - JK + KL = JL - JK
KL = JL - JK
x - 9 = 2x + 8 - (2x - 2)
x - 9 = 2x - 2x + 8 + 2
x - 9 = 10
Often times, we would solve for x, but considering x - 9 is what we wanted the solution to anyway, we don't need to bother solving for x.
KL = 10