I honestly don’t know how to solve this...Would I just be dividing 18 in half? Because it looks like the line is split evenly down the middle.

Answer:
12
Step-by-step explanation:
This is one of those “not to scale” pictures. We know that BC is twice as long as AB.
AB=x
BC=2x
AC=18
x+2x=18
3x=18
x=18÷3
x=6
AB=x=6
BC=2x=2(6)=12