Explanation
Mechanical advantage (MA) is the ratio by which effort force relates to resistance force. If both forces are known, calculating MA is simply a matter of dividing resistance force by effort force:
\(M A=F_{r} \div F_{e}=9 f t . \div 18 f t .=0.5\)
NOTE
In this case, the mechanical advantage is less than one meaning that each unit of effort force results in just 0.5 units of resistance force. However, a third class lever like this isn't designed to multiply force like a first class lever. A third class lever is designed to multiply distance and speed at the resistance at a sacrifice of force at the resistance. Different lever styles have different purposes and multiply forces in different ways.