If you weigh 180 lbs now and are 18% bf, that means you're carrying 157.5 pounds of lean and 32.5 pounds of fat.
So if you wanted to get to 9% bf, that would simply be half of what you have now - 16 lbs or so.
But I think what you're really asking is if I weigh 180 lbs now and am 18% bf, how much would I weigh if I dropped my bf to 9%?
In that case, the formula is: Target Weight = LBM (lean body mass) divided by (1 - desired body fat as a decimal).
In other words, 157.5 divided by 1 - .09 = 173 lbs.
What that's saying is if you train and eat properly, and lower your bf to the 9% range, your body weight should be close to 173 lbs.