Hey there! I'm happy to help!
We see that at AB is halfway across our line. We see that AC is the total line.
So, to find the other half, we have to subtract AB from AC, which will give us the value of BC.
We first will subtract our x values.
3x-x=2x
And then we subtract our numbers.
-6-5=-11
So, BC is 2x-11.
Have a wonderful day and keep on learning! :D