Because the objective of this course is to teach someone who already speaks English how to speak German, using symbols to answer a question posed in German should be fine.
However, as solopipe notes, using the symbols to provide an answer dass soll auf Deutsch werden, ist nicht so gut. (Even though German uses the same digits and symbols for basic, if not all, mathematics as does English.)
Normally, it would be "four and four make eight" (not make
s) because you have a compound subject--i.e., more than one--which is plural, so the verb is conjugated for the plural.
Cf. "Jack and Jill carry [not carries] buckets of water." Jack carries one. Jill carries another.
Interestingly, one could use the singular verb "is" ("Four and four is eight") because in that case the two items are considered together as a single item. However, one will also commonly hear "four and four
are eight", which may be more correct. (Cf. "Jack and Jill are a couple.")
Apparently both are correct; here's a short discussion on the issue.
I take the stance that it's not a compound subject because if it were, you could apply the subjects separately to the sentence-- i.e., "Four makes eight" and "Four makes eight," which is obviously nonsense. But both "makes/is" and "make/are" are fine.
I'm not sure using the addition problem is the best choice for analyzing the grammar by splitting the subject, because you are correct: "four makes eight" is not true, unless you include the other component and say something like "four makes eight, with another four."
In fact, I'm not sure that the test of applying the subjects separately is the right test for whether a phrase is a compound subject. For example, in "Jack and Jill marry each other", "Jack and Jill" is a compound subject, but "Jack marries each other" is nonsense.
The real test of whether something is a compound subject is the definition:
"A compound subject is two or more individual noun phrases coordinated to form a single, longer noun phrase."