So this means W(-3x) = 3x. So it's equal to 3(-3x), which simplifies to be -9x. Whenever you have a function, it looks like f(x). f doesn't have to be the letter, it can be a, b, c, or like in this case, W. It just means the output of what W(x) is if x equals the number in the parentheses, which is -3x in this case.
So if 32 oz is 20% more than the average bottle, you multiply 32 by .20 which gives you 6.4. This means that the 32 oz bottle is 6.4 oz more than the regular bottle. Furthermore, 32 - 6.4 = 25.6 so that's how big the typical bottle is.
