I'm going to go further than saying "this isn't a math problem" to saying "the solution presented is incorrect." The problem could be phrased as a symbol logic problem--can you arrange these symbols in a way that creates a mathematically valid equation. But, the problem is currently stated as a mathematics problem--it was on a "math" test and relies exclusively on the language of mathematics. What people seem to forget here is that mathematics is a different language--if you are going to state a problem in that language, you are implying a solution in that language.
I previously noted, in the language of math, we imply multiplication all the time when two symbols are placed adjacent with no operator--"ab" is "a times b." Therefore, in the language of mathematics, two discrete numbers adjacent to one another should be multiplied together. The "solution" presented relies on concatenation. The operator used to denote concatenation in mathematics is a double bar, since concatenation is useful in some work with prime numbers and other number theory--in other words, "5 || 4 = 54" is a correct mathematical expression. However, the problem implicitly restricts you to combining certain numbers using addition--denoted by "+"--and multiplication--denoted through parentheses. The problem does not allow you to use concatenation through parentheses, and therefore a solution relying on parentheses is wrong.
This math teacher should be de-certified.