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YOUR ANSWER: 3 is the subject and = is the verb.
Let's look at an english sentence. If we say, "John is my son," John is the subject of the sentence. However, if we turn the construction around and say, "My son is John," then my son becomes the subject of the sentence. The same thing happens when we move the parts of a mathematical sentence from one side of the verb to the other.
We have already seen that in the mathematical sentence 3 = 2+1, 3 is the subject. If we change this construction around so that it reads 2+1 = 3, then 2+1 is the subject. Of course, there's no law against having a compound subject in mathematics any more than there is in english.
Now return to page 3 and choose the other answer.