Because you can solve this problem with a system of linear equations does not mean you should.
So I'm with all the respondents who have found the title of the question a little misleading.
However, it is interesting that this question should make you think of a system of linear equations and thereby make you question its relevance to grade 3 students (which I gather must be a young age), because, in my humble and uninformed opinion, this question should make the student think of down-to-earth and practical ways of approaching the problem, rather than mechanically reach for a rigid tool.
This question, it seems to me, should teach students to 1) see that the problem is not about rectangles, but about any kind of discrete objects, like marbles or apples and 2) since 24 is a very manageable number, the student should solve it by trial and error playing with small objects and/or dots on a sheet of paper. After finding a solution, the student should think about whether there might be more than 1 solution. They may also think about numbers other than 24, think about numbers other than 6, groupings other than 2, etc..
I doubt very much that the student is expected to compute the discriminant of a system of linear equations.