All these people complaining about the notation need to switch on their brains. Obviously, the parentheses (which were probably a circle or oval field on paper, anyway) are not the problem. How do I know that? It's simple: the students filled them in! They understood perfectly that the blank space was supposed to be filled in with a number.
But just as in a grammar problem you have to choose the right word to put in the blank to make the sentence correct, in a mathematical problem you have to choose a number that results in a correct formula, and that's where they failed. They didn't understand that a formula with an equal sign (an equation) is correct if and only if the two sides have the same value. This is what TFA means by "the meaning of the equal sign".