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.
