Step-by-step explanation:
there is not much to explain.
we have a piece of line (LM) that is 8 long.
and then we add another piece of line (MN) that is 9 long.
this is basically the same type of question like "when you have 3 apples, and your mom gives you 2 more apples, how many apples do you have now ?"
for the apples it is 3+2 = 5.
for the lines here
LN = x = LM + MN = 8 + 9 = 17
that's all.
